Eintracht Frankfurt midfielder Marco Fabian is "on the ropes" after suffering a knee problem in training, coach Niko Kovac has told reporters.
Mexico international Fabian, who missed parts of the second half of last season with back pain, was forced out of a training session after a problem with his left knee.
"You can clearly see that he's not happy with the situation. He's a bit on the ropes right now," Kovac said.
"Marco has difficulties with his knee. He took part in two training sessions, and then the problems started."
Fabian returned late to training following his involvement in the Confederations Cup, and Kovac added: "He didn't have any [problems] in his three weeks with the national team, or at least I am not aware of them.
"Maybe he already had problems but got through, and he now realises that it's not possible. I don't know."
Although the midfielder has undergone examinations, the club have yet to trace the cause of the problem and fear doing so could take a while.
Kovac said: "The body is pretty complex and you can't just look at it locally."
Frankfurt kick off their domestic season with a cup match at TuS Erndtebruck next week.