The Switzerland goalkeeper is in his second season with the Nerazzurri.
And Hamann told Blue Sport: "I have to say that he is one of the best goalkeepers in Europe at the moment. Last year he was exceptional. And obviously I am happy because he is a very good goalkeeper.
"When they were in Munich last summer, they obviously had some problems in the management and Thomas Tuchel was under pressure. None of that probably helped him at that time."
He continued: "Everyone also needs confidence in your centre-backs and the centre-backs or the whole team needs confidence in the goalkeeper and he has achieved that with exceptional performances. The most difficult thing is often when you are only needed two or three times in the game.
"You always have to be focused, those are often the most difficult situations. It is easy to hold up well when you take 30 shots on target and the goalkeeper saves 27. The trick is that when you are in goal for the big clubs, you have to be there once or twice when they need you and he does that exceptionally well."
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