The former Germany captain says that the playmaker is "not the same player" he was at Real Madrid and is worried about his performances heading into the World Cup.
Arsene Wenger broke the club's transfer record to sign Ozil for 42.5 million pounds last summer, but the Germany international endured a difficult first season at the Emirates Stadium, despite scoring seven goals and providing 14 assists.
The playmaker was criticized following a penalty miss during the Champions League quarterfinal clash with Bayern Munich and struggled to recover his form, with Ballack claiming “he has not been the same player” since leaving Santiago Bernabeu.
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"It’s difficult to say where his head is going to be at the World Cup," he is quoted as saying by 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.”
Germany drew with Cameroon in an international friendly at the weekend, with Ozil deployed in the hole behind the striker, but the 25-year-old had a negligible impact on the game and Ballack admits he was “concerned” by the performance.
"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 added.