Atletico hammered Barca in the first-leg at the Metropolitano 4-0 on the night.
Afterwards, Flick said: "We didn't play well in the first half, we didn't play as a team. The gap between us was huge. We didn't press. And we learned a big lesson in the first 45 minutes.
"The end was better. We have two more halves and we'll fight. If we win 2-0 in each half... we'll need our fans at home.
"I'm not disappointed with the team. I'm happy with the team's season. We've had a lot of injuries. Losing is also part of the game. It was a painful defeat, but I'm proud of the team. We have to learn from this.
"They were hungrier to score. And that's what I want. We didn't do it in the first 45 minutes. But the team is young, and I'm not making excuses."
Asked of this was his worst night as Barca coach, Flick added: "No, the worst was against Inter. Now we have a chance to come back. It won't be easy, but we'll try."