Former Germany midfielder Michael Ballack is concerned that Mesut Ozil has lost his way since joining Arsenal last year.
Ballack, who played for Germany in the 2002 and 2006 World Cup final tournaments, believes his compatriot is not in the best of shape mentally ahead of this year's finals in Brazil.
And the ex-Chelsea, Bayern Munich and Bayer Leverkusen player is worried that the £42.5m Gunners' signing from 2013 is currently underachieving.
Ballack told the Daily Mail: "It's difficult to say where his head is going to be at the World Cup.
"I saw him play for the national team at the weekend and I was not surprised, not scared, but a little bit concerned, by his performances.
"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.
"But he is not a leader on the pitch like a captain. He is now at an age where he is no longer a young talent or a promising player.
"He is a fantastic player who still has to find his role and his responsibility for the team. When he was criticised for his performances, he looked surprised, he looked lost.
"As one of our best players he has to take more responsibility, and we expect more from him at the moment.
"Arsenal is such a big club and a move like that can make a big impact. But sometimes he looks as if he carries too much on his shoulders."