Bayern Munich midfielder Toni Kroos is battling flu and could miss Saturday's north-south derby against Werder Bremen, the club said on Friday.
Bayern, whose coach Jupp Heynckes was ill earlier this week and missed training, have dropped to third place after losing three of their last five league games, including the last two.
"He [Kroos] trained on Thursday but was sent home by doctors due to flu symptoms and told to stay in bed," the club said.
Bremen's top striker Claudio Pizarro could be fit for the match after training with the team following a knee injury.
Pizarro has scored 11 league goals this season.
"The knee is fine. If the coach says 'yes' then I will travel to Munich," he told reporters after training on Thursday. "But I do not know if it will be good enough for the full 90 minutes."
Werder are in fourth place, three points off leaders Borussia Dortmund and two behind third-placed Bayern.
Borussia Monchegladbach coach Lucien Favre has warned his players they will be up against "one of Europe's top teams" on Saturday when they host champions Borussia Dortmund in a top-of-the-table clash.
Dortmund lead the standings ahead of Gladbach, who also have 29 points after a stunning start and have yet to lose at home this season.
"It was clear that Dortmund would struggle at the start of the season but now they have found their balance again and I think they are one of Europe's best teams," said Favre.
Dortmund have won seven of their last eight league games.
