Barca won 6-0 thanks to doubles from Fermin Lopez, Robert Lewandowski and Raphinha.
Afterwards, Flick stated: "I'm very happy with the performance, from the start. We played with intensity and together. We saw how the starting eleven played superbly and the rest of the squad also raised their level. It's a new beginning after the break. Marc Bernal's return is also emotional."
The win was achieved without Lamine Yamal, who was sidelined with a knock.
Flick continued: "There's always a first chance for everything. It doesn't matter who's missing. Frenkie (de Jong) and (Alejandro) Balde weren't there either. We focus on each game. Now we're focused on Newcastle. We're very happy to get this win."
Fermin was outstanding on the night, leaving his coach pleased.
Flick said, "He had a very good game, but not just him. The whole team enjoyed it. When the transfer window closed, I was happy because it allowed everyone to focus on playing. Everyone did a great job. Eric, Cubarsí... they played very well in defense, the midfield also did well... it's similar to what I saw at the stadium last year.
"We have to stay calm. Today he took a big step forward, he handles himself well with and without the ball, he's won balls back... he shows confidence, and I think he can improve even more. We have to stay calm, but maybe he shouldn't do a nutmeg and give the ball away... but he's at a fantastic level."
Flick chose to start Roony Bardghji ahead of Raphinha and explained: "It's a decision. Roony is training well and has a good mentality and attitude. Raphinha was coming off a long journey. He has a great mentality. We only have eleven players; there are many. That's all I can say. Roony's attitude was fantastic. He wants to learn, to keep learning."