Schalke forward Klaas-Jan Huntelaar faces eight weeks on the sidelines after suffering a knee injury as his team completed a derby double over Borussia Dortmund.
Huntelaar, 29, had put Schalke in command when he added to Julian Draxler's goal to give them a 2-0 advantage with 35 minutes gone.
But he later collided with goalkeeper Timo Hildebrand as he tried to clear a ball into the Schalke area and tore the medial collateral ligament in his left knee.
Taking to Twitter after the match, Huntelaar had tweeted: "S*** happens..!! Still 2-1…!!!!!"
After it was confirmed that the Netherlands international faces a lengthy spell on the sidelines, Schalke skipper Benedikt Howedes said: "We'd like to wish the Hunter a good recovery and hope he'll be available again soon."
Schalke general manager Horst Heldt said: "It is a shame that Klaas-Jan Huntelaar suffered an injury. He played great. That's the sad part of today."
The setback means the Royal Blues, who face Galatasaray in the Champions League on Tuesday with their tie poised at 1-1, are short of strikers.
Earlier this month, they cancelled Brazilian forward Edu's contract to enable him to move to China, while Romanian forward Ciprian Marica is still recovering from knee surgery.
It means Finland international Teemu Pukki is Schalke boss Jens Keller's only fit and experienced striker, but Howedes said: "We will compensate somehow. In our game at Mainz, we played Jefferson Farfan up front."