Celtic v AC Milan
Celtic still stand a faint chance of snatching second spot in Group H and, while that may be unlikely, a win over Milan would certainly strengthen their claim to third and a place in the Europa League knock-out rounds. On paper, it might seem that Celtic have little hope against the Serie A giants, but the same was said before the last meeting between these two teams at Parkhead in 2007, when the Bhoys ran out 2-1 winners thanks to a last-minute winner from Scott McDonald (now at Millwall). An impressive home record in Europe offers hope to Neil Lennon's side, who will also be encouraged by Milan's disastrous away form, with the Rossoneri failing to win a single match on the road so far this season.
Borussia Dortmund v Napoli
A clash that was dubbed the hipster's dream before the first round of fixtures has become a crucial game for both teams on matchday five. Following their surprise defeat to Arsenal, Dortmund must avenge their opening day loss to Napoli to stand a realistic chance of progression. A 3-0 home defeat to Bayern Munich on Saturday was far from ideal preparation, however, with Dortmund missing three-quarters of their first-choice defence as Neven Subotic (out for the season), Mats Hummels and Marcel Schmelzer spend time on the sidelines. Napoli have been going strong in Serie A, winning nine and losing only three of their 13 games so far, but two defeats in a row offer hope to Dortmund ahead of Tuesday's crucial fixture.
Arsenal v Marseille
Arsenal's victory in Dortmund has put them in pole position in Group F and they can qualify for the last 16 on Tuesday if they beat Marseille and Dortmund fail to win against Napoli. The Gunners' opponents have failed to pick up a single point so far and have only scored four goals, although two of those strikes came in a hard-fought 3-2 defeat to Napoli in their last outing. This clash is seen as a home banker, but Arsenal cannot afford to be cocky against Marseille, who possess talented players in Florian Thauvin and reported Chelsea target Giannelli Imbula. However, the news that Andre Ayew will miss the game with a knee injury has handed a late boost to the Gunners, with Arsene Wenger perhaps tempted to mix up his team ahead of Saturday's trip to Cardiff.
Bayer Leverkusen v Manchester United
Although Manchester United lead Group A after the first four matches, they could have made life a lot easier for themselves with a win against Real Sociedad at the start of November. The Premier League champions looked rather toothless in that 0-0 draw in San Sebastian and they have only scored twice in their last three Champions League fixtures since finding the net four times in the opening game against Leverkusen. Sami Hyypia's side failed to give an honest showing of themselves on that occasion and, after climbing into second, they will be looking for revenge on Wednesday. A 4-0 thrashing of Shakhtar Donetsk in October underlined Leverkusen's goalscoring threat and David Moyes will be eager to emphasise the danger of Stefan Kiessling and Sidney Sam, with the duo notching seven goals each so far in the Bundesliga.
Juventus v Copenhagen
At the start of the tournament no-one would have predicted that Juventus would find themselves below Wednesday's opponents after four matches, but that's exactly where the Old Lady lie following three draws and a defeat to Real Madrid. Copenhagen, led by former Wolves manager Stale Solbakken, have only scored three goals so far, but that has been enough to gain four points following a 1-0 win over Galatasaray in their last outing. The race for second spot in Group B is still wide open but Juve cannot afford another slip-up at home ahead of their trip to Galatasaray in the final game, which could well see the two teams face off in a winner-takes-all scenario.
Matt Stanger - he's on the Twitter.