Barca were good. United tried to press them and win it back by chasing and pressurising defenders right up the field, but Barca don't give it away very often even when faced with that. United lost the desire to chase after the first goal went in and then just played into Barcelona's hands completely. I don't think Fletcher would have made a difference.
Barca on the other hand knew that if they chased the ball and pressurised the United players all the way up the pitch they would get it back quickly.