Bayern Munich midfielder Sebastian Deisler will be out of action for an undefined period after sustaining knee injury, the German champions' official website reported on Wednesday. The German international has injured his right knee in the Bavarian club's training on Tuesday; he will fly to the United States to undergo surgery to know the full extent of the injury. "We don't have a precise diagnosis, so we can't say how long we'll be without Sebastian," Bayern coach Felix Magath said. Deisler, who has undergone five operations on his right knee in the past eight years, will miss his side's Bundesliga clash against Schalke on Sunday as well as Germany's friendly game with United States next Wednesday. The 26-year-old's injury could hamper his chances to participate in the World Cup finals in Germany as the tournament starts on the ninth of June.