ASM won 2-1, with Barca seeing Eric Garcia sent off on 10 minutes in France. Lamine Yamal canceled out Maghnes Akliouche's opener for ASM before George Ilenikhena's winner.
Flick later spoke to reporters, insisting he was satisfied with the performance.
Overall game:
"The game has changed completely. We tried, we defended with passion. We made some mistakes but we tried to score goals. We had some chances but they deserved to win."
Defeat:
"We lost, okay, but now we have to come back stronger, recover. There is a match on Sunday. We have to give our best, fight for the team. We lost this match, but we will continue fighting."
Monaco:
"They were very good, but that's normal. They are very fast. We tried to defend ourselves."
Reorganise the team:
"The changes were not late, we defended with a lot of passion and we had opportunities. We tried to be compact in defence, together. The game changed with the expulsion. But I see positive things, we attacked and defended as a team."
Concerned?
"No, I'm not worried. I have to analyse it, it's my job as a coach. I saw the players sad in the dressing room but now we have to recover. We will analyse the game, we will talk to the players to correct the things that were not done well. After the red card, the game has changed."
Lewandowski, Lamine and Cubarsi
"Lamine tried hard today, he worked hard. But yes, the triple substitution was also with Villarreal in mind."
First defeat:
"We saw a red card in the first 10 minutes and that has changed everything. These things happen, we have to accept it but I think we are strong enough to play in the Champions League. There are seven games left and we will win many games and achieve our objectives."
Ansu Fati:
"I already said in the last press conference that he can play in various positions. I have spoken to him and he prefers to play as a left winger. I agree, but he can also play as a centre forward, he is very good with the ball. I have seen a very different Fati to the one who was at Brighton. You can see the quality in him. I think he needs more time, we have to look after him."
Worst match?
"When you've played five games and lost, you can say it wasn't the best game. But the performance, beyond the score, I think the beginning was good, but not exactly how we wanted to play."