United is a team that revolves around player roles unlike Barca that focuses on players. Barcas O is at times spotless, but Uniteds D and hard work is unmatched by any other team. As I see it there are two possibilities of United losing. One, Barca scores early, United cant get their O going. Two, United scores early goes into D mode (like Chelsea) but gets overwhelmed and someone slips up, resulting in a goal in the back or two. Fletcher is important, having him gone sucks. As for Barca Abidal wont play, however Poyol is back. Park impressed against Arsenal, nobody works as hard as him.
To sum it up. As long as United plays like they did against Arsenal with tons of players helping in the back, getting their passing game on and are able to finish (which they usually do), there's no chance Barca can take this.