Sir Alex bills United vs Real Madrid as "the biggest game you can get in Europe", so ahead of Wednesday's visit to the Estadio Santiago Bernabeu and next month's clash at Old Trafford, we asked United fans around the world for their thoughts on how the games will pan out...
How would you like to see United approach the match?
Peter Bolton, from Timperley, near Manchester, replies: Sir Alex and his staff should do their homework on Real Madrid but not worry too much about them. After all, Jose Mourinho's mob couldn't even win their Champions League group and they're miles behind in La Liga, while we won our group with two games to spare and we're top of the Premier League. We're definitely good enough to knock Madrid out - and they've been knocked out at this stage, the last 16, in three of the last five seasons. Let them worry about us, and let us treat this like any other European away match.
How do we stop Cristiano Ronaldo?
Casper Heiselberg, Copenhagen, Denmark: It will obviously be difficult to tame the monster we created, but I am sure Sir Alex knows exactly how to deal with him. My suggestion is to close him down quickly, be tough on him and instruct our full-backs not to be too attack-minded to avoid missing any of his great runs. He’s a strong player, but I am sure the likes of Rafael and Patrice Evra can cope with him.
Who will be United’s main threat to Real Madrid and how do we make the most of him?
Zeinab Fawaz, Beirut, Lebanon: That depends on which formation Sir Alex will use. The formation in itself can be our secret weapon and to make the most of our attacking threat, we should go with 4-4-2 in my view. Playing wingers will keep Real Madrid's fullbacks occupied, allowing them little chance to venture forward.
We've got brilliant match-winners in Rooney, Valencia, Nani, Kagawa... but if I had to choose one main threat, and we're really blessed to have two exceptional strikers, I'd go with Robin van Persie.