Barca must overturn a 2-0 deficit at the Nou Camp next week.
"We have not done things right to win, that's for sure," said the midfielder.
"Whether the players are tired or it is the system, the game against Milan will be like a Cup Final and we must come out from the start as if our lives depend on it.
"I'm confident we can knock them out.
"It is not that we cannot play, it is that we are trying and other circumstances beyond us are in play, like our opponents. It's a matter of moments, feelings.
"In these two specific games, which were important, the opponent has won. We lacked a little bit extra, but not a precise thing.
"When a team does not work it is a matter of everything. From back to front and front to back. There are opponents who play differently and you have to adapt.
"When we are in a good dynamic, we create more chances. In these two games we missed that. We must recover the ABCs of football because that is what has led us to become the team that we are today."