Its difficult to pick a team tonight for United. 4-4-1-1 has been successful for them and Hernandez+Rooney are a fantastic partnership. But going with 2 in midfield against Barca is suicide and Rooney as the third midfielder means he won't be able to influence the game as he would if he was upfront. Tough but I'd probably start 4-5-1, try and keep it tight and then hope Barcelona tire, bring on Hernandez and Nani and counter them as Barcelona attack as Arsenal did in the 2-1.
But I don't think United's midfield is strong enough, they have a lot of heart and fight but ultimately I think Xavi and Iniesta will have a lot of fun against whoever United put out in the middle. Arsenal beat Barcelona because they had a strong midfield and United haven't got 2 players of Fabregas or Wilshere's quality in the center.