The Gunners have lost three out of their last five matches.
Wenger admits: "Maybe we have lost something (without Van Persie) but we need to look at that in the longer term.
"Van Persie could make a goal with half a chance that is for sure, I do know that. And when you lose 40 goals somewhere (it is harder).
"I'm not saying the players are fragile but you could see in the defeat against Schalke on Wednesday night that we played all right in the first half but slowly we dropped. Is it physical or confidence? It is difficult to know."