His form has dipped in recent weeks and Wenger admits it will take the German striker time to get used to the graft required to survive in his new surroundings.
Wenger said: "He started well but he works very hard. In recent games he has been a bit less fresh. He's not used to working at that level of intensity.
"When you come to England it's a shock. In England, you have Wayne Rooney who works his socks off. Everybody works hard.
"There is no room for anybody to work less or you don't exist.
"It's quite interesting to analyse. For example, the game against Man United, where we were not very good, the intensity of the game physically was unbelievable.
"When you look at the numbers after the game it was quite un-believable. If you have just one or two players that don't do that, then you don't exist at all."