Robert Lewandowski requires groin surgery but is yet to decide when he will have the operation, interim Bayern Munich boss Hansi Flick has confirmed.

German publication Kicker reported on Monday that Lewandowski could go under the knife during the international break, with Poland having already secured their place at Euro 2020.

The 31-year-old striker has been in sensational form for Bayern this season, scoring 14 goals in 10 Bundesliga appearances and 20 in 16 outings in all competitions.

Flick, who was charged with taking charge of the first team following Niko Kovac's dismissal in the wake of Saturday's 5-1 thrashing at the hands of Eintracht Frankfurt, confirmed Lewandowski is struggling with a groin issue but is yet to set a date for an operation.

"We were a bit surprised to read that in the press but it's true. He has problems but he doesn't have pain," said Flick at a news conference to preview Wednesday's Champions League meeting with Olympiacos.

"He'll need to undergo surgery at some point, but he will talk to our medical staff. He will think carefully about interrupting his current spell.

"It's his decision and we will respect it because he's a super-important player for us. We will have to wait and see when it makes sense and he'll miss the least amount of games."

Flick confirmed he would play Thomas Muller and Javi Martinez against Olympiacos and used his first training session on Tuesday after the players returned from a day off to work on the defence.

"The goals we have conceded recently have not been Bayern-like. That's what we have to focus on," said Flick.

"We practiced a couple of things we want to see changed and the team did a really good job. I can't change everything – 90 minutes is not enough to change a whole team, but it's about the attitude and maybe changing one or two things tactically, emphasising one or two things differently.

"I want the team to be active and taking the initiative. We need intensity in defence when we are with and without possession.

"Since I've been here I've been fascinated by the talent in the squad. They have enormous potential to perform and we're not seeing enough of that."