"It looks a serious hamstring injury. We assessed the hamstring but Theo wanted to stay on the pitch," The Sun quotes Wenger as saying.
"I think he should have come off straight away because, the second time, it was a real hamstring problem."
Walcott attempted to play through his injury before Wenger was forced to replace him with Gervinho on 60 minutes.
"Now it'll be tough for him to come back until the end of the season. It'll be 21 days out, minimum.
"But he should be okay to play for England in the summer, he will just have a good rest. I don't think he will play for us again this season."