Barca won the quarterfinal 2-1 thanks to goals from Lamine Yamal and Ronald Araujo. A late strike from Javi Moreno set up a nervous finish for Barca, though they survived.
Flick said afterwards: "It's a Cup match... It was a difficult game. We suffered. They played very well. We created many chances, but we need to finish better. It was a very even match, but I think we deserved the victory.
"We need to finish our plays better, with more conviction. We have to improve. We played on Wednesday, Saturday, and today... we can't say anything about the mentality or the attitude. I liked my team. You have to win these Cup matches, and we did, and that's why we're happy."
On Araujo, the coach added: "He's taking it step by step, that's what's important. The goal will help him gain confidence and feel better about himself. We support him. We're with him. He's doing well.
"In the end, he was tired. Today was a difficult day because he hadn't played that much. But today he took another step forward. It was an important goal for him and for us."