The Blaugrana were fancied to beat the Rossoneri on Wednesday night, but goals from Kevin-Prince Boateng and Sulley Muntari ensured the Liga giants went home empty-handed.
The Camp Nou centre-back hopes he and his team-mates can learn from what was a flat display at San Siro, and refused to blame any external factors for their defeat.
"We are Barcelona and we have to fight back," he told the Catalunya side’s official website.
"We had the match under control in the first half but we didn’t play well in the second. We fell apart after they scored the opener, it was out of control. We have to improve and learn from this.
"It looked as though they were coming into the match as victims but we know this club, the history they have. There are no excuses - the pitch, the referee, nothing - we have to work hard to fight back."
Andres Iniesta was one of the illustrious Barca attackers who was ineffective against Milan, and he hopes the team can recuperate ahead of the second leg on March 12 in Barcelona.
"I don’t think this game was lost because of small details, I think we lacked freshness, we didn’t create chances, and they were well organised," the Spain international said. "This is the Champions League.
"We have to think about the return leg and make sure it turns out differently, we have to try and attack from the first minute.”
Sergio Busquets was not left downhearted after the loss and has faith that Barca can turn the tie on its head in three weeks’ time.
"We’re very eager for the return leg so that we can turn this game around," the defensive midfielder stressed.
"It’s time to fight back. When you lose you always have to think about what went wrong and we have to prepare for the match as best we can."