Bayern Munich sporting director Hasan Salihamidzic admitted the leg injury sustained by Franck Ribery on Sunday "does not look good".
Ribery appeared to jar his knee when he trod on top of the ball during the second half of Bayern's deflating 2-2 draw away to Hertha Berlin.
The former France star hobbled off the pitch and was in too much pain to be quickly examined by medical staff.
Ribery's injury compounded a difficult first match for interim boss Willy Sagnol, who watched the champions cough up a two-goal lead for the second time in as many Bundesliga fixtures.
"Franck has a lot of pain. We hope that there is nothing bad, but it does not look good," Salihamidzic told Sport Bild.
"The doctor is investigating him. He could not examine him at first, because he had so much pain."
Wishing you a speedy recovery, ! October 1, 2017
Having taken over from the sacked Carlo Ancelotti on Thursday, Sagnol appeared set to celebrate a debut victory when Robert Lewandowski added to Mats Hummels' early opener.
But a string of missed opportunities came back to haunt Bayern as the hosts turned the match on its head with quickfire goals from Ondrej Duda and Salomon Kalou.
"We needed to play with more concentration and discipline," said Sagnol.
"We had a lot of chances and only scored two goals. That is a disappointment. We should have been more efficient."
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