mr_renoogA diagram to highlight a potential weakness of Man Utd's 4-2-3-1 against Barcelona's 4-3-3.
With Messi dropping deep as per usual, Barcelona would have a diamond in midfield against Man Utd's trio. With Carrick playing deep and keeping an eye on Messi, Giggs up against Xavi, and Rooney keeping goal-side of Busquets, Iniesta would be left relatively free.
Park of course can drift in from the left (as he usually does), but that would only help to double up on Xavi, and wouldn't solve the Iniesta problem. Carrick could play a bit more right-central and help to track Iniesta, but that would involve Giggs covering the space in front of defence (i.e. Messi), which is not an ideal scenario in the slightest.