The midfielder became the Gunners’ all-time record transfer when he joined from Real Madrid for £42.5million at the beginning of last season.
However, the playmaker has come under plenty of criticism following some less than inspired performances at club level, and Ballack, who played for Germany at the 2002 and 2006 World Cups, is concerned about what Ozil will bring to Brazil later this month.
Former Chelsea star Ballack said: “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.”