Hansi Flick has revealed Thiago Alcantara is struggling with a muscular injury just four days before Bayern Munich visit Borussia Dortmund for Der Klassiker.
Spanish midfielder Thiago completed 90 minutes as leaders Bayern beat Union Berlin 2-0 last weekend, picking up where they left off in their first Bundesliga game since the resumption after coronavirus.
However, Flick has confirmed the 29-year-old has since picked up a muscle problem that puts his availability for Saturday's clash with Eintracht Frankfurt in doubt.
Though Flick has not ruled Thiago out of that match, it is a quick turnaround for Tuesday, when Bayern face their nearest rivals Dortmund in a game which could have a huge say in the eventual destination of the title.
Discussing Bayern's injury problems at his pre-match news conference, Flick told reporters: "With Serge [Gnabry], it looks good. With Thiago, we have to wait. We hope that we have both with us. We will catch up. Otherwise the situation is like last week."
Thiago has played in all but three of Bayern's 26 Bundesliga matches to date, the Bavarians beginning this round of fixtures four points clear of Dortmund.
They have kept three successive clean sheets too, despite not having centre-back Niklas Sule available since October because of a knee injury.
Germany international Sule is still keen to be involved before the campaign finishes, though Flick is cautious.
"We don't want to rush anything," he added. "Niklas has expressed the wish to play this season. We have to see how stability in the knee allows."
One Bayern defender who could be involved is Lucas Hernandez, however Flick said the club record signing will have to bide his time.
France international Hernandez was signed from Atletico Madrid for €80 million (£71.5m) prior to this season, but the World Cup-winning defender has started just seven Bundesliga games.
Knee and ankle injuries have not helped the 24-year-old's cause, though he has been an unused substitute in the previous three Bundesliga games either side of the break enforced by the pandemic. David Alaba, who has been converted to a centre-back because of injuries, and Jerome Boateng have instead been at the heart of Flick's defence.
"David and Jerome have shown excellent performances in central defence," Flick said. "We have the best defence in the league. Of course, Lucas is plagued by injuries. Now it is important that he shows excellent performance in training.
"I am very satisfied with his mentality, he gives always 100 per cent, but it is also clear that if a chain of four works well, you as a coach rely on this security. But it is clear that we need every player in our squad."