United's central defenders are the key because against Barcelona it is easy for them to become spectators, watching the game taking place in midfield.
Key battle: United should follow Arsenal's lead and use Rio Ferdinand to subdue Lionel Messi
Barcelona play without an orthodox striker, as Lionel Messi is constantly dropping deep into the mini-pitch that his side create near the centre circle. They can get as many as six men in a small area and just concentrate on keeping possession. It is sacrilege for them to give the ball away.
That creates a problem for United: their midfielders quickly become outnumbered, with no-one to pick up Messi, Pedro or whoever else floods into the centre of the pitch while Rio Ferdinand and Nemanja Vidic are left with nobody to mark.
That is an uncomfortable feeling as a central defender.
You like to know who you are looking after, not sit watching the game worrying that someone will suddenly come running at you at high speed.
All this means that Barcelona will have the lion's share of possession and United will eventually be punished.
But there are ways to prevent this. As long as Wayne Rooney picks up Sergio Busquets, then there are enough players (Michael Carrick and Ryan Giggs) to pick up Xavi and Andres Iniesta. Park Ji-sung and Antonio Valencia must concentrate on dealing with Barcelona's bombing full backs.
But to stop Messi making it four against three in the middle, one of United's central defenders must follow him into Barcelona's mini-pitch. It is something Laurent Koscielny did particularly well for Arsenal in the second half of their win over the Catalans at the Emirates and Lucio did brilliantly for Inter Milan last season in their semi-final victory.
They went after Messi, tracked him and won the ball more often than not. It is an unorthodox approach and would mean United abandoning their traditional defensive shape, but it is the best way to prevent being outnumbered and have two redundant centre halves.
Little master: Rio Ferdinand can only watch as Lionel Messi heads the ball past Edwin van der Sar in the 2009 Champions League final
Ferdinand will find this role easier than Vidic because he is more mobile but whoever goes with Messi must do it with conviction and believe they are going to win the ball.
Of course, dealing with Messi is just part of the problem; you also have Pedro and to a lesser degree David Villa, who like to drift inside into that central area and that causes a whole new problem.
Normally you would expect United's full backs - Rafael and Patrice Evra - to mark the opposition wingers but when Pedro and Villa drift inside so deep it would be too risky to do that, especially when you have Daniel Alves or Carles Puyol surging forward on the overlap.
Evra and Rafael will have a good view of what is happening and if they see their wingers start to drift inside unmarked, they will have to tell Park and Valencia to stop picking up the Barcelona full backs and follow the wingers.
Then Evra and Rafael will be charged with looking after Alves and Puyol. They should basically switch who they are marking - but it is not easy as it is all happening so quickly. But if they can do that and a centre back can track Messi, they can stop Barcelona outnumbering them and therefore dominating possession and the match.
Key to all of this is a tough selection dilemma for Sir Alex Ferguson. He could be tempted to play Anderson because he has been in excellent form and has the engine to get up and down and chase the ball, which you need against Barcelona.
Whoever plays in central midfield will need to close down Xavi and Iniesta. But I think Ferguson should opt for Carrick and Giggs. Carrick is so good for Javier Hernandez because of his early and accurate passes and they bring the best out of each other. And United need Giggs's experience.
He has shown this season how important he is, even if he hasn't got the engine of Darren Fletcher or Anderson. Once United have sorted out how to win the ball and stop Barcelona dominating, they have the ability to exploit Barcelona's weaknesses.
Central defenders Gerard Pique and Javier Mascherano are not the fastest and United have bags of pace in Hernandez. Barcelona will press the United defenders when they have the ball but if Sir Alex's side can slip passes in to Carrick and Giggs, they can then hit quick, long balls to Hernandez in the channels, where there will be space because of the full backs' obsession with getting forward.
From there, Hernandez can cause all kinds of trouble and Rooney has the strength and pace to get up and support him, as does Valencia.
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Explore more:People: Alex Ferguson, Daniel Alves, Ryan Giggs, Gerard Pique, Javier Mascherano, Antonio Valencia, Michael Carrick, Lionel Messi, Darren Fletcher, David Villa, Nemanja Vidic, Andres Iniesta, Wayne Rooney, Park Ji-sung, Patrice Evra, Rio Ferdinand, Laurent Koscielny Places: Barcelona