Borussia Dortmund centre-back Nico Schlotterbeck will not play again this season after tearing the meniscus in his left knee, the club said on Monday.
"Nico's absence hits us very hard," said Dortmund sporting director Sebastian Kehl in a club statement.
"We hope he will get fit again quickly and will receive maximum support from us on his way back."
Schlotterbeck, who missed Saturday's 4-1 Bundesliga win at Freiburg due to suspension, has been one of Dortmund's best players in a poor season for the club.
Champions League finalists last term, Dortmund have fallen to eighth in the league and have only an outside chance of qualifying for Europe's top competition next season.
Schlotterbeck has 20 Germany caps and has established himself as a starter in Julian Nagelsmann's side, who will play Portugal in the Nations League semifinals in Munich in June.
The defender's hopes of returning in time for the Nations League look slim, however.
German media report Dortmund are looking to extend Schlotterbeck's deal, which expires in 2027, amid interest from abroad including Premier League leaders Liverpool.
Dortmund play away at Barcelona in the Champions League quarterfinals on Wednesday, followed by a trip to domestic rivals Bayern Munich on Saturday.