Fans turned on the coach as Barca were hammered in traditional Joan Gamper Trophy season curtain-raiser.
Flick said: “I know what this team can do. It wasn’t our game, we haven’t been good, but in Valencia we will do it differently. I’m sure we will have a different performance. The time to lose is now. Better than during the season.
“When you lose 0-3 it’s not easy for the fans, but the atmosphere in Montjuic has been fantastic. When we play fast, the public’s reaction is perfect, so we have to provoke that emotion from the pitch.”
Flick also said: “We had the opportunity to go ahead on the scoreboard and then the game would have been different. During the first half we did not play as fast as we would have liked. The speed and passing have not been as good as in previous games, we lost a lot of balls there.
"We did not control the match and that has allowed Monaco to improve. In the second half we were better, but a mistake scored us 0-1 and ended up triggering this huge result.
“We can play much better. We have five days to prepare for the game against Valencia, to improve the pressure and our game. I’m sure we’ll do better on Saturday.”