Muller believes Ozil is suffering from fatigue.
He said: "It's not easy for Mesut because you have to play so many games - I don't know how many games he has played for Arsenal without having a break.
"In the Bundesliga we have just had a month off.
"In England it's very hard to be a professional footballer.
"You have many more games than we do in Germany. But I hope Mesut has good games for Arsenal because he is a friend of mine and I know how good he can be."
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