Hoffenheim's Pirmin Schwegler could face a significant lay-off after tearing ligaments in his knee at the weekend.
The midfielder picked up the injury towards the end of his side's 4-4 draw with Mainz on Sunday and tests on Tuesday revealed the damage was much worse than originally thought.
"It's extremely bad news for us but in particular for Pirmin," said Hoffenheim coach Julian Nagelsmann on his club's website. "He's been in good form in recent weeks and is now going to be missing for an undetermined time.
"I have the utmost faith in our medical department, that they will get him fit again as quickly as possible."
The 29-year-old Switzerland international has featured in two of Hoffenheim's three Bundesliga fixtures this season.
He missed four games of last season after partially tearing knee ligaments, although the current injury appears to be more serious.