After victory over Sevilla last night, Wenger said: "We have to keep our feet on the ground.
"We have to improve in a lot of areas, one or two times defensively against Sevilla we weren't clean enough.
"We have improved from two years ago without a doubt but it is the beginning of the tournament and there are two competitions in the Champions League, the groups and the knockout, and you need the best players available for the second part of it.
"The challenge of the season is to show how far we have come. The tactical strength is growing and when we were under pressure we didn't panic."