The Spain international has conceded that it will be tough to overcome Massimiliano Allegri's men in the second game.
"We let in two goals that could have been avoided," he told reporters. "The result is a very bad one, but this team has never given up.
"In the second half we did not play as we normally do, whereas Milan had the game of their dreams.
"We know it's not going to be easy to get through to the next round, but we will try.
"We have to be more united than ever."