Bayern Munich loan signing Serdar Tasci was being examined for suspected concussion after suffering a head injury in his first training session on Wednesday.
Bayern sporting director Matthias Sammer said Tasci had collided with a teammate. He missed his scheduled presentation press conference.
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The Bundesliga champion signed the former Germany defender from Spartak Moscow until the end of the season on Monday, the last day of the winter transfer window, as cover for injury.
Defender Javi Martinez underwent surgery to remove damaged cartilage in his left knee on Tuesday and will be sidelined for about four weeks. Germany international Jerome Boateng is out for several months with a groin injury.
Tasci, who joined Spartak in 2013, spent seven seasons at Stuttgart, helping the side win the Bundesliga in 2007.