Benfica are now above Celtic in the group, and would stay in second if they manage to win at Camp Nou due to having the better head-to-head record with Lennon's side.
That's it. Benfica cling on for a 2-1 win that means the final qualification spot will be decided in the last week of group matches. Celtic are still in the driving seat, though, as they entertain Spartak Moscow at home. Benfica, meanwhile, have to head to Barcelona.
And another near miss for Celtic! A looping cross from the left breaks at the back post to Watt, who drives the ball right across the face of goal. Two team-mates are there but neither can throw out a foot and direct it into the inviting net.
Commons' ball is decent enough and after the initial header clear, Kayal gets the chance to lash the ball goalwards. He catches it sweetly but gets underneath it, sending his attempt way over the bar. That might be the best chance Celtic are going get to snatch an equaliser.
Salvio off, Jardel (a centre back) on. Here comes the free kick...
Almeida fouls Samaras, then makes absolutely certain of receiving a booking by smashing the ball somewhere towards the moon. Chance here for Celtic to load the box as we move into stoppage time. If only Mulgrew was still on the field...
Forster keeps Celtic alive! Gaitan found Cardozo and after slipping the ball through Ambrose's legs, giving him a clear sight of goal, the beanpole striker sees his left-footed strike tipped over by the visiting goalkeeper.
Samaras slides in a superb pass inside the right full-back for Watt, who cuts back inside but can't get enough on his strike at goal. Good opportunity goes begging for Celtic - how many more will they get in these closing minutes?
Lennon is furious when a foul is given against Watt right in front of him on the touchline. It's becoming a bit scrappy, and Benfica have no interest in doing anything more than hanging onto this one-goal lead.
Wanyama penalised again, and the referee decides it warrants a yellow card this time around. Nothing naughty in the foul, perhaps given as much for an accumulation of misdemeanours. That booking means the midfielder will now miss what looks set to be a crucial final group game against Spartak Moscow.
Wanyama is punished for a foul, but again it looked a little soft. Still, Celtic break from a poor Luisao pass and had Commons put a little more on his pass to the over-lapping Lustig, the full-back would have been in on goal.
Stunning save from Forster. Cardozo curls in a free-kick that the Celtic keeper does superbly well to tip onto his bar. Looked a dubious foul in the first place on Gaitan, but replays showed he was clipped by Ambrose on the edge of the box.
Ledley heads off for Watt - he scored 11 minutes after coming on against Barcelona. He's got about the same amount of time to try and do the same tonight. Remember, a score draw here in Lisbon sees Celtic through to the knockout stages.
Matic - who has been solid in the middle of the park for Benfica - is replaced by Maxi Pereira. Celtic, meanwhile, seem to be readying super-sub Tony Watt down on the touchline.
Melgarejo becomes the first Benfica player to see a card after blocking off Lustig down the right.
Lima is heading off the field, with Nicolas Gaitan taking his place.
After Wanyama had forced Artur to make a save low to his right at the other end, Salvio clips the Celtic bar with a dipping attempt that had Forster concerned. Great game now!
Cardozo goes down in stages on the edge of the Celtic box, and the referee is having none of it. Good job for Celtic, too, as the forward had a team-mate unmarked to his left who would have had a clean run in at goal.
Garay succeeds where Lima failed just seconds before - a right-wing cross is nodded back across by his fellow centre-half Luisao and the former Real Madrid man smashes in a right-footed volley.
What a let-off for Celtic! Lima gets in space down the left channel and plays in a delightful cross towards Salvio. The Benfica man is a yard in front of Matthews but completely misses his kick when no more than six yards from goal. Perhaps could've gone with a diving header on the first bounce, but certainly should've got something on it.
Wilson does just enough to put Cardozo off his header. Celtic's back four are doing all they can to keep back the Benfica tide right now. You wonder if Lennon might be thinking of using his third and final substitution to help give some extra support in the middle of midfield. Hooper is getting more and more isolated.
Forster gets down to his left to keep out Lima's low drive, a comfortable save for the goalkeeper but a further warning that Celtic still have a long way to go yet.
Samaras does well to win a free-kick on the left. Is the Benfica storm starting to die out?
Artur Moraes this time does deal with Commons' set piece, albeit choosing to punch, rather than catch, the ball.
A rare period of play in Benfica's half sees Kayal win a corner on the right. Celtic will gladly take it, if for no other reason than it gives their defence a bit of a break.
Here comes the second change for Celtic - Brown, who has missed seven games this season with a hip problem, makes way for Kris Commons. Samaras takes over as skipper.
Brown's game is almost up - Celtic are readying a replacement for their struggling skipper.
Sky Bet update: Benfica are building up the pressure on Celtic, but the Bhoys excelled in the same situation against Barcelona - they're now 7/1 to nick this one tonight.
Matic can't resist but have a go when the ball sits up nicely from 20 yards out after he'd chested it down. He catches it cleanly, only gets too much on his left-footed attempt.
Another Ambrose clearance, another Benfica corner coming up. Perez before that had wowed the crowd with a nutmeg on Kayal - I hope he called it.
Luisao has a half chance to put Benfica in front again, though it was never easy to steer a bouncing knock down underneath the bar. Forster watches it safely drift over the top.
Celtic getting pushed further and further back. Ambrose blocks a cross out for a corner.
Matthews produces a 'boot-iful' block to concede a corner. The home crowd want a penalty and although the ball struck his arm, it was always tucked into his body.
Pressure mounting now on the visitors...
Tremendous defending from Matthews, I can't state that enough. He anticpated the danger and retreated behind his goalkeeper, providing an extra line of cover to knee the ball clear. He's had an excellent game (so far) at left-back.
Lima makes Ambrose look silly down the left channel, skimming the centre half to give him the chance to cut in at goal. Forster comes out but can't stop the shot, meaning he's indebted to the covering Matthews for blocking the goal-bound attempt.
Lustig is at full stretch down the right to stop the dangerous John from getting free.
Ambrose is there again to head away the corner and Celtic soon get the chance to push up the field when Brown wins a free-kick, Melgarejo using him as a ladder to win a header.
Ambrose plays safe, heading a right-wing cross over his own bar. This is exactly how the first half started, Benfica looking to stretch their opponents out wide and feed balls into the box.
Ledley becomes the second Celtic player to be booked - it was a good, lunging tackle but you simply don't get away with them these days. In my Sunday morning league he'd have got a round of applause and a pint bought for him in the pub afterwards.
Matic was excellent first half for Benfica, though his attempted pass to put in Cardozo is a tad too heavy. I say too heavy, it cleared the advertising board on the full.
Kayal has come on as a replacement in the last three Champions League games Celtic have played.
Change at the break - Kayal is coming on in place of the ill Mulgrew. Big loss for Celtic, not so much in open play but because of Mulgrew's brilliant set pieces.
Joseph Griffin has tweeted in - he fances a 2-1 Celtic victory. That'd be typical wouldn't it, waiting so long for an away win in Europe's premier club competition, then getting two in the space of a few months!
The players are returning to the field - if it finishes 1-1 remember, Celtic are through.
Watching the replays of Celtic's equaliser, I may have over-estimated in the distance Samaras was from goal. Make that a maximum of two yards, not three. Another case for why zonal marking doesn't work - space doesn't score goals.
John Collins in the Sky Sports studio: "Celtic have to play better in the second half, take better care with their passing."
So Celtic supporters, can you do it? Send in your thoughts to Rob.Lancaster@bskyb.com, or tweet to @SkySportsLanny.
Benfica 1 Celtic 1 - The Scottish champions have one foot in the knockout stages. Another 45 minutes of the same and they will join Barcelona in qualifying from the group.
Forster fortunate to get a free-kick after coming for the far-post corner, Luisao put him under pressure but didn't seem to do too much wrong. The referee didn't agree, however, meaning Celtic's goalkeeper was spared after dropping the ball.
Celtic are putting their bodies on the line! Not one, not two, but three blocks on the edge of their area, the last of them by Wilson sending Salvio's effort wide for a corner.
Forster gets his leg in the way of John's left-footed drive after Lima's pull back from the right had come to him at the far post. Not too unlike John's chance that he scored from earlier, only this time he had to have a go with his other foot.
Almeida cuts in from the right wing but fires his left-footed shot well wide of Forster's goal. Half time approaching and it's so far, so good for Lennon's Lisbon Lions.
Salvio gets teed up on the edge of the Celtic area by Perez, only to blast his shot way, way over the top.
For once Mulgrew's left foot lets him down, failing to clear the man at the front post with his cross. It nearly results in a chance at the other end, Benfica breaking quickly and Matic's shot forcing a corner at the other end of the field.
Benfica clearly want to have another go at defending a corner from the right when Almeida needlessly gives away a set piece under no pressure whatsoever.
Samaras is the first man into Mr Kassai's book when he's shown a yellow for declaring his frustration at being penalised down the left wing.
It's a dangerous-looking ball in from John, but Celtic deal with it well.
Samaras does well to come back and do his defensive duties...only he's given away a free-kick in a dangerous position out near the left touchline.
All Celtic now - they've managed to push their defence higher up the field and are now winning the ball in better positions.
Samaras gets another sight of goal - this time from 20 yards out on his right - foot but badly drags his shot wide.
Celtic fans are singing in the rain now - after a shaky start they've steadied the ship and seized upon their first real chance at goal. Again, it was all about Mulgrew's wand of a left foot.
A say easy task for Samaras, he did momentarily worry that he'd managed to nod it into the floor too close to his own toes, the ball eventually looping up into the roof of the net.
Wonderful ball in from Mulgrew, curled into the back post to find a wide open Samaras, leaving the forward with the easy task of nodding in from about three yards out. Dreadful, dreadful goalkeeping by Artur Moraes, who watched that drift almost across in front of his nose.
Samaras! It's 1-1 in Lisbon!
Celtic win a corner - here's a good chance to pose a threat to their hosts, particularly with Mulgrew's delivery.
Chance for Benfica! Cardozo ghosts in behind Ambrose to get on the end of a threaded ball, teeing up perfectly on his left foot. However, the forward clearly didn't study trigonometry, attempting to drive it in Forster's near post rather than taking the better option of going across goal.
Samaras is down...but the referee punishes the Celtic forward for handling the ball as he went to the turf.
Celtic again win the ball well in midfield - Wanyama intercepting a pass - but the pace of their counter attack soon disappears and a foul is eventually conceded by Brown.
Aidean in Dublin: "I believe even though we have allowed Benfica to score, we still can come back and win, Our performances away v Spartak Moscow and Barcelona have shown us we can do, but we need the Spirit of Lisbon 67 more now, I feel 2-1, Samaras and Hooper."
The home crowd are fuming when Perez is blocked off by Wanyama, not a man you want to run into at any time. It's left the Benfica man with a war wound, a bleeding mouth forcing him to leave the field for treatment.
The game has gone a little quiet since the early Benfica goal - Celtic won't mind that too much, though, as an equaliser and a point would be enough to see them seal a place in the next round.
More Benfica possession - they've pushed their full-backs high up the field and it's penning Celtic's two wide midfielders deep.
This will be the venue for the Champions League final in 2014.
Samaras slips in a lovely ball down the right that Wanyama tries to get to, only for Luisao to come rushing across and deal with the danger. Impressive piece of defending - the home fans give their skipper a standing ovation for it!
Hooper flagged for offside running down the left channel. He's onside, and by a distance, but the official seems to have flagged because Samaras was off. Easy to get those two confused I suppose...
The referee seems unsure who to give a free-kick to when there's not one but two Benfica players on the floor. He slowly walks over, realises it's on the edge of the box and then awards the foul to Celtic.
Sky Bet update: Celtic are now 16/1 to come from behind and win in Lisbon.
Up until then, Celtic had been chasing shadows a little bit. And I don't mean Hank Marvin and the rest of the band (ask your dad). They've been brave with two up top, but their midfield is having to do a lot of work without the ball.
First real chance to break for Celtic as Samaras is picked out by a long, raking ball to the right. The play is then worked back across the other side of the field, only for Mulgrew to cough up possession with plenty of team-mates suddenly in the box.
Salvio comes off worst after a clash of heads with Ledley. Looked painful, but he slowly rises to his feet like a heavyweight boxer who has taken a heavy right hook.
Wilson is in the right place to this time hammer away a cross from the left. Benfica are looking to get at Celtic down the wings, though falling behind hasn't stopped their large group of supporters from singing.
Lovely strike from John with his right foot, though replays show it took a little nick off Lustig before going under the right arm of Forster. Celtic will be disappointed on two counts - firstly they failed to stop the cross from the right, then they fluffed the chance to get the ball away to safety.
GOAL! Celtic fall behind early on, it's John in the right place to slam home a loose ball in the box.
It never rains, but it pours. Well, at least that's the case in Lisbon today.
Surprising number of empty seats inside the ground - clearly the Benfica supporters don't rate their side's chances of making it out of the group.
Skipper Brown doesn't wait too long to leave his mark on Lisbon, though Melgarejo seems to make a meal of the slightest touch. Celtic must keep their discipline.
Early warning from Cardozo, making his first Champions League start of the season tonight, as he fires a left-footed effort from long range over the bar. All came about after Samaras was too easily nudged off the ball on the halfway line.
Lennon has been positive, to say the least. From the early set-up it appears Hooper and Samaras are both starting up front, meaning a 4-4-2 formation.
And we're underway! Remember, a score draw would be good enough to take Celtic through - they are already at least certain of a place in the Europa League next year.
Viktor Kassai of Hungary is our referee - he was the man in charge of the 2011 Champions League final.
The teams are out, the Champions League music has been played, it's time for the action to get underway in the Stadium of Light!
STAT ATTACK! Benfica have had 26 shots on target during their European campaign this year - but another 36 have missed! They have, however, not conceded a goal in their last five games.
Ray Wilkins' thoughts on Gary Hooper: "If you can score goals, you can score goals." Nothing more really needs to be said, does it?
Pre-match odds from SkyBet: "Following their unforgettable defeat of Barcelona in their last Uefa Champions League outing, we've had plenty of punters backing Celtic at 5/1 ahead of this battle for second place in Group G. The Portuguese hosts are 1/2 for the win, while the draw (100/30) has been the least popular outright bet. Tony Watt, whose goal proved to be the winner against Barca, is forced to settle for a place on the bench, and he's 10/1 to score last. Gary Hooper starts and is 8/1 to score first, but Benfica duo Oscar Cardozo and Rodrigo Lima are joint favourites (100/30) to break the deadlock."
Lennon: "We've come here to get a positive result, whether it be three points or one, (The result from earlier) doesn't change our mindset. We're very, very strong in the front six and our back four has played well in recent games. They're going to get tested."
Neil Lennon's thoughts pre kick-off: "It's great that we have European football ater Christmas, but obviously we'd like to be in this competition. We know we're in for a very difficult evening but the lads are up for it."
So Celtic looking strong tonight - Scott Brown returns after missing the win over Barca while Joe Ledley and Gary Hooper have shaken off injuries to start. It means Kris Commons, Beram Kayal, and Tony Watt have dropped to the bench.
Send in your views on the game, Lennon's team selection and also score predictions to Rob.Lancaster@bskyb.com, or via a tweet to @SkySportsLanny
Provan: "Everything has swung Celtic's way - Barcelona did them a turn earlier, now Celtic have to do themselves a favour tonight. I think they believe they can defend at this level, they've been far more disciplined. They know what their roles are."
Former Celtic player Davie Provan is in Lisbon to commentate on tonight's contest - "Celtic will want to get the job done tonight. They have an insurance policy with a home game against Spartak, but I'm sure Neil Lennon will want to get through tonight."
In case you've buried your head for the past fortnight, or appeared on 'I'm A Celebrity, Get Me Out Of Here', then you know that Celtic managed to beat Barca last time out in Europe. Having also recorded their first away win in their Champions League history in Moscow earlier in the campaign, confidence is high ahead of this game at Luz Stadium this evening.
Barcelona's victory over Spartak Moscow in the early kick-off means a score draw will be good enough for Neil Lennon's men to seal qualification.
Good evening. Welcome to coverage of Benfica v Celtic. Lisbon is already a special place for the Scottish side, but tonight it could take on even more importance if they can qualify for the knockout stages of the UEFA Champions League.