United had a promising start to the game but struggled after Samuel Eto’o’s goal.
The Welshman told Sky Sports: “We are disappointed with the result and the performance. We didn’t turn up tonight. They turned up, they played some great football. We didn’t really.
“We didn’t play the football that we are capable of and that is disappointing. Credit to them, they deserve it tonight.
“We started the game so well and that’s what we aimed to do. We aimed to put them under pressure and we created a couple of chances.
“We created chances and we probably created more chances than Barcelona.
“I think, with these two teams, whoever goes ahead obviously has the upper hand because they keep the ball so well. Unfortunately they went ahead.
“At times Barcelona can make you look silly because they keep the ball so well. At times we maybe chased it and didn’t keep our shape as we should have.”
He added: “We have got no right to win every competition and league. You have got to earn the right.
“I think there is always a good reaction to disappointments and I think we will come back bigger and better. Hopefully we will be back next year.”