Bayern Munich coach Hansi Flick said it was too early to determine the extent of midfielder Joshua Kimmich's injury that overshadowed Saturday's 3-2 win at Borussia Dortmund.
The Germany international has been pivotal to Bayern's success under Flick but was helped from the pitch in obvious distress after twisting his knee in a first-half challenge at Signal Iduna Park.
"He'll fly home with us and be examined - it's too early to say the extent of the injury," said Flick.
"He's a very important player for us and with his mentality, he isn't someone you can replace like for like."
Kimmich is also a concern for Germany ahead of the friendly against the Czech Republic on Wednesday before Nations League games against Spain and then Ukraine in the following six days.
Bayern earned their fourth straight win over Dortmund as Robert Lewandowski scored his 11th goal in just six league games to move top of the Bundesliga.
After Marco Reus gave Dortmund the lead, David Alaba blasted home a free kick to make it 1-1 at the break before Lewandowski struck with a superb header early in the second half.
Lewandowski, the European player of the year, could have finished with a hat-trick after the VAR ruled out further efforts either side of half-time.
Leroy Sane came off the bench to grab Bayern's third 10 minutes before the final whistle while Erling Haaland scored Dortmund's late second goal.
Norwegian striker Haaland was annoyed at his own performance after converting only one of his four chances.
"I have to score more goals, if we don't put our chances away at this level, then we won't win," said the 20-year-old.
"They are the best team in the world, but we have to work harder to take the next step to be as good as them."