The tie is evenly balanced; United will feel they have a slight advantage after their 1-1 draw in Madrid, while the Spanish side will be full of confidence after back-to-back wins over Barcelona.
I'm really looking forward to seeing how it unfolds.
I was delighted for United after the first game and I thought they had a good advantage with the away goal; they were very disciplined, they played well and put in a top class performance to get the result they did.
But since then I've been to Barcelona to watch Real play the Catalans in the Copa del Rey; it was Real's full strength team against Barcelona's full strength team - and it was one of the best away from home performances I've ever seen.
Real totally frustrated Barcelona and then totally dominated them on the break with Ronaldo and co marauding at the Barcelona defence; I came home more worried for United than I was after the first leg.
This game is perfectly set up for Jose Mourinho and his favoured tactics of stifling teams and then beating them on the break.
Real need a goal and that may alter the shape of the tie, but looking at performances Mourinho's teams have come up with over the years - from Porto to Inter Milan in the Champions League - it's going to be a mammoth task for United.
In terms of the match itself it could be a game of chess with both sides not prepared to over commit or it could be one of those nights when passion and adrenaline and the levels of commitment turn it into a free-flowing attack-minded game.
I'm not sure it would suit United if it was like that; they need to slow the pace of game and start cautiously and not try to finish it off in the first 20 minutes.
If United did pull it off and progress it would be one of Sir Alex's greatest results; they could not be playing a world super team in better form - and they have to recognise that.
David de Gea came of age in the last game and his performance could determine their fate as much as any player - but I can't stress how difficult it's going to be.
If they played Real six weeks ago it might have been different but Real are in cracking form and I can see Real either taking it into extra-time and going through or getting the victory in 90 minutes.
My hunch is Real to go through - but I'd love to be wrong.
This week's other Champions League games
Borussia Dortmund got a vital equaliser right at the death in the first leg against Shakhtar Donetsk in Ukraine and that swung the tie in their favour.
On paper it looks the same as the Manchester United game in terms of what Dortmund have to do - but their task is easier as they're not playing Real Madrid.
Dortmund are a more than decent side at home; we saw in the group stages when they beat Real Madrid, Ajax and Manchester City on their own patch how good they can be. They looked fantastic at that stage of the tournament and I fully expect them to progress here.
Unfortunately, Celtic's tie appears to be over already; the 3-0 defeat at home to Juventus was heart-breaking for them because they would have been hoping their supporters would have inspired them to a result at Celtic Park which would have given them a chance in Italy.
I hope Celtic score first in this one because that would then make it a bit wobbly for Juventus - and the longer the game goes on without anyone scoring the more comfortable it will be for the Italian side.
But fair play to Celtic because they've had a couple of amazing performances so far in this tournament. It may be too much to ask for them to pull off another incredible result in Turin but they can look back on some fond memories.
I hope their trip to Juventus isn't in vain though, if not in terms of progressing but in terms of learning from the experience of facing one of Europe's big sides.
In years to come these Celtic players don't want to be talking about the time they were knocked out by Juventus, they want to be talking about the big teams they took on over a sustained period of time.
To do that they need to learn from this experience, but in terms of the match, I can only see a Juventus win.
The final Champions League match this week sees PSG host Valencia. The home side hold a 2-1 lead and, despite a shock defeat to 10-man Reims in Ligue 1 at the weekend, you'd expect the French side to finish the job at home.
They won't have Zlatan Ibrahimovic in their side and that's a blow - he's been in wonderful form but is paying the price for a needless late sending off in the first leg of this encounter.
However, they will have David Beckham in their squad and the former England captain will be relishing the chance to play Champions League football for the first time in a few years.
There were plenty of people criticising his move to Paris, saying it was all about his image. But he's made the PSG fans smile over there and I expect he'll be smiling too come the final whistle on Wednesday.
I'm going for PSG to progress.