The Spanish league leaders are looking to bounce back from defeat at Real Sociedad and a draw at Celta Vigo in their last two outings.

Barcelona let a two-goal lead slip in the final stages against Celta on Saturday and Flick said his young team, which has defeated Bayern Munich in Europe and rivals Real Madrid in La Liga, needed to take another step forward.

"It's about eliminating the mistakes, that's important," Flick told a news conference Monday.

"We have a lot of things to do better with the ball and that's the focus."

The German coach said Barcelona should have defended better after going down to 10 men against Celta Vigo when Marc Casado was sent off, to protect their two-goal advantage.

"We have a young team, I think it's the youngest team in La Liga. It's not always a highway, where you go at high speed, you have to take care in some situations," continued the coach.

"We have to learn that -- when we lead 2-0, with the red card we have to defend much better than we did."

Barcelona are sixth in the Champions League table with three wins and one defeat from four matches, but Tuesday's visitors Brest are one point ahead of them in fourth, despite sitting 12th in Ligue 1.

Brest scored an eye-catching 4-0 win at Salzburg among other positive results and the visit to face Barcelona gives them the chance to take a huge scalp.

"They are doing really good, they deserve their 10 points and it was good how they played against Salzburg," said Flick.

"They are in a flow in the Champions League, they struggle a bit in the league. They're a good team."

Barcelona will be without teenage winger Lamine Yamal, still recovering from an ankle injury, while Flick said left-back Alejandro Balde is also not fit enough to start.

Barcelona defender Inigo Martinez also showed his frustration with the team letting their lead at Celta slip.

"It was hard to digest, it was a game we had under control to some extent -- we let crucial points get away from us," Martinez told reporters.

"Physically we've got a very difficult opponent ahead of us, they're doing very well in the Champions League.

"Tomorrow if we come out relaxed it could cost us dearly."

The 33-year-old centre-back said if Barcelona were at their best, even well prepared opponents would struggle to beat them.

"Opponents obviously study Barca games and analyse them, as we do with our opponents," he continued.

"It's all about how we press and our intensity. If our press is good it's very difficult to beat us."