mr_renoogAs dicussed here...

and here...

...I believe a 4-2-3-1/4-4-1-1 formation would be problematic for Man Utd as it would leave key players free for Barcelona.

Therefore I believe a 4-1-4-1 would serve Man Utd best, as it would enable them to mark Barcelona's main creative threats man-for-man.
-Carrick would protect the space in front of the back 4 (Messi's zone)
-Fletcher and Park would provide good work rate in front of Carrick, helping to harrass Iniesta + Xavi. As a bonus they are both comfortable drifting to the flank.
-Rooney would be assigned to track Alves

Busquets would be left free to spread the ball around among the fullbacks and centre midfielders, but that may be a price worth paying in ensuring that the creative quartet of Alves-Xavi-Iniesta-Messi have no time on the ball.

Attacks would primarily be focussed down Barcelona's left-hand side (where their pressing is slightly less intense), utilising the pace of Valencia, the poaching instincts of Hernandez, and the diagonal runs of Rooney drifting into the box from the left.