The German, now settled in London after his £18million summer move from Bayer Leverkusen, said: "I think it's normal that he was frustrated.
"We had a bad match and we didn't play well. It's only normal that the manager wasn't very happy with us.
"When we do something wrong or don't play well he always tells us straightaway. But he also does it directly - face to face with us.
"That's good because after that, after he's told you, you know you have to do something to prove him wrong, to prove we are a good team and that we can play well."
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