The rumours come in the wake of speculations linking the forward to a move to Manchester United and Newcastle United.
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While Kovac dismissed the reports, he talks up the quality of the German stating he has the potential to become a world-class talent.
"I firmly assume that he can become a truly great footballer if he adjusts certain things. But for that, he has to believe me a little more,” he told the media.
“He has to work on his accuracy, his meticulousness. He has been given huge talent, now it's about work, changing details and nuances that will move him forward. To do that, he has to use every minute of training.”