Goals by Kevin-Prince Boateng and Sulley Muntari put Milan firmly in the driving seat ahead of the return leg of their last-16 tie on March 12.
A return to Primera Division action should provide some comfort for Barca, who enjoy a 12-point lead over Atletico Madrid heading into Saturday's match with mid-table Sevilla.
Midfielder Xavi admitted 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."
The squad's unity was at least boosted this week by the return of defender Eric Abidal.
The Frenchman has spent the best part of a year recovering from a liver transplant he underwent last April.
He returned to full training with the squad at the end of December and will now be available for selection for Barca's remaining games, starting against Sevilla.
Xavi said: "It's the best news of the year. Seeing Abidal, how he trains and gives it everything and that he's now able to return to playing is a source of motivation.