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 La Liga 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.
"It's the best news we could have had."
Sevilla may only sit 10th in La Liga but they ran Barca agonisingly close in the reverse fixture in September.
Los Rojiblancos were leading 2-1 at Ramon Sanchez Pizjuan heading into the final minute only to see Barca snatch all the points with quickfire goals from Cesc Fabregas and David Villa.
With the European berths just five points away, Sevilla still have everything to play for and will be eager to capitalise on the Catalan club's European blip.
The visitors insist that they will not adapt their system to counter Barca's threat, with defender Alberto Botia ruling out the option of man-marking Leo Messi.
"Many things have been said about how to mark Messi, but they have so many good players that if you just focus on him, the others will cause you damage," Botia said.
Botia, who came through the Barcelona academy as a youngster, added that the confidence gained from Sevilla's good performance against Tito Vilanova's men earlier in the season will carry through to Saturday's match.
"Barca are difficult opponents but we made things difficult for them here and we should use that as inspiration to get something from the game," he added.