Tuchel says the former Tottenham defender has "exceeded expectations".
"I am very satisfied with him," Tuchel said.
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"He has exceeded all expectations. He is an extremely important piece in our puzzle. He organizes, is vocal, and that was something we didn't have much of before.
"He always helps the person next to him, he is a top class guy and a very good player. He is being rewarded for his hard work at the moment."