Barca won 3-0 at San Moix thanks to goals from Raphinha, Ferran Torres and Lamine Yamal, with the hosts having Manu Morlanes and Vedat Muriqi sent off before halftime,
Flick said afterwards, "I didn't like the game. They're three important points, but I didn't like it. After a 2-0 deficit and two red cards, I think the team played at 50 percent, and I didn't like it. We can do better.
"I don't like them to let their guard down. We had to control the ball and the game. We have to score. Playing at 50 or 60 percent against nine men isn't possible. We have to play faster. We have to improve some situations."
On Marcus Rashford's debut as a second-half substitute, Flick said: "It wasn't a good half for Rashford, because there was no space. The players played at 50 percent."
However, on goalkeeper Joan Garcia, he added: "Joan had a great game. He can continue like this."