Barcelona coach Hansi Flick has said picking Wojciech Szczęsny ahead of Iñaki Peña was one of the toughest decisions of his career.

Szczęsny, 34, signed for Barça in October, coming out of retirement to join as a free agent after first choice goalkeeper Marc-André ter Stegen was ruled out for the season.

The former Arsenal and Juventus stopper had to wait until January to make his debut, starting a Copa del Rey tie against Barbastro, but he has now been in goal for five of Barça's last seven games.

"It was a tough decision," Flick said in a news conference on Tuesday of the switch from academy graduate Peña to Szczęsny in recent weeks.

"Being No. 1, the goalkeeper is a special position so this is, yes, I can say yes, it's one of the toughest decisions I ever made."

The decision has generated debate after Szczęsny was sent off in the Supercopa win over Real Madrid, committed two mistakes in the 5-4 victory over Benfica last week and gave away a penalty, which was later overturned by VAR due to a prior foul, in the 7-1 rout of Valencia on Sunday.

Flick refused to go into the reasoning for his selection, explaining that he didn't want any praise for the former Poland international's qualities to be taken as criticism of Peña.

"When I say good things about [Szczęsny] it means maybe [it's reported] as weakness for Iñaki and I don't like this," he added.

"My job is to make the decision on this position. For me, Szczęsny is an experienced player, of course, but he also has a good personality.

"Both of them are great goalkeepers, but we decided for Szczęsny at this point. Nothing else. Also, in the past, Iñaki made a good job, but now I decided like that while thinking about the team."

Flick was speaking ahead of Barça's final game in the Champions League group phase, which is against Europa League holders Atalanta at the Olympic Stadium on Wednesday.

Barça have already sealed their passage to the round of 16, but a seventh straight win in Europe would guarantee they progress in at least second place, potentially giving them a more favourable rout to a hypothetical final.

"The Champions League is the toughest competition to win," Flick said. "There are a lot of great teams. For us, its important to win tomorrow because we want to finish second. For the next steps in the competition, it's a good situation.

"But it's not easy to play Atalanta. They play one against one over the whole field. When you build up, you are always under pressure. And when they win the ball, the transition is very fast.

"They play always one contact. We have to be ready for that. I have the highest respect for [coach Gian Piero] Gasperini and the unbelievable job he's done. They play a unique style in Europe and are brave in away games."