Xavi says Barca were hurt by the defeat at the San Siro and insisted there are lessons to be learned.
"It was a long journey back for the players and coaching staff," Xavi said. "The result is very, very bad for us.
"We have to realise what we did wrong. We have to correct our errors and not have doubts about ourselves, that would be the worst thing. We have to be more united than ever."