Schalke left-back Sead Kolasinac could be out for at least six months after suffering an anterior cruciate ligament teat in his right knee.
The Bosnia-Herzegovina international was forced from the field 16 minutes into Saturday's 2-1 defeat at the hands of Hannover in Schalke's opening Bundesliga match of the season.
Subsequent tests revealed a tear and Kolasinac is expected to undergo surgery on Tuesday.
The 21-year-old, who featured twice for his country at the World Cup in Brazil, made 24 league appearances for Schalke last season.
Kolasinac was quoted during the close-season as saying he had been offered a move to the Premier League with Manchester United.
However, that transfer failed to materialise, with the player eventually remaining at the club where he has spent his entire professional career.comments