Here's a good preview from Jonathan Wison: How best for Man U to combat Barca?
I agree with most of what Wilson says with one big exception: I think he's dead wrong with the final bit about how to neutralize Alves. Alves isn't going to pick up the ball at midfield and run at defenders, beating them with 1v1 dribbling moves. He's going to be in the final third when he gets the ball; so, not only isn't it imperative to put a defensive-minded winger on him, it's entirely useless to do so.
The quality of defender isn't going to influence Alves; but the quality of attacker on the other side may. Alves will put in his defensive work if forced to do so. History has sown that he will play further downfield (at least marginally) against a strong attacking threat. As such, I think Ferguson should play Nani on the left side, rather than Park. Park would be more useful in the center, where his defensive industry would pay greater dividends. Park has also become a more creative attacking threat over the past year and has played in the center of a 4-2-3-1 for Man U in big games this season.
Of course, to do this, Rooney would need play alone up top. He wouldn't have Hernandez's pace to stretch the defense and open up room for him in between the lines. It's that quality of Man U's game that may in fact prove to be more important than any of the defensive tactics at issue. If I'm Ferguson, I give up on the idea of shutting Barca out or getting anywhere near them in terms of possession. I dance with who brung us, and that is high-energy attacking. I put the best passers on the field to get the ball to the best attackers. That means a 4-4-2 with Scholes and Giggs (maybe Fletcher) together, Nani and Valencia on the outside for the good crosses and 1v1s, with Hernandez and Rooney up to (obvioulsy with Rooney frequently dropping off).
That means Barca scores one at least, probably two. But I take my chances that I can score two or three with that lineup.