Arsenal midfielder Mesut Ozil has faded as the season has gone on and his German compatriot Muller believes it is because he is not used to such an intensive schedule.
The Bundesliga has only recently returned after a month off, during which time Arsenal played eight matches, and Muller has some sympathy with his Premier League conuterparts.
Muller scored Bayern's second in a 2-0 win at Emirates Stadium on Wednesday in the first leg of Bayern's UEFA Champions League last-16 tie with Arsenal.
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 any 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."
Muller has also suggested that the lack of a winter break in the Premier League is damaging England's chances at major tournaments.
He said: "When England get to World Cups they look a bit tired.
"The body can recover but the head's another story."