"It's difficult to say where his head is going to be at the World Cup," the former captain told the Daily Star.
"He has done well for Germany and he played well during the qualifying campaign but in the last few months - since his move to Arsenal - he has not been the same player, in terms of his confidence, that we used to see in the national team.
"I don't know whether the Arsenal move has affected him because he was untouchable and in Germany we still expect him to be one of our best players.
"As one of our best players he has to take more responsibility, and we expect more from him at the moment. Hopefully, when the World Cup starts, we will see a different player."
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