Hansi Flick has revealed Thiago Alcantara is struggling with a muscular injury just four days before Bayern Munich visits Borussia Dortmund for Der Klassiker.
Spanish midfielder Thiago completed 90 minutes as leader Bayern beat Union Berlin 2-0 last weekend, picking up where it left off in its first Bundesliga game since the resumption.
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.
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"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."