However the Arsenal manager believes Walcott will recover in time for England's assault on Euro 2012.
Wenger said: "It looks like a serious hamstring."
The Gunners boss added: "First we assessed the hamstring, he wanted to go on and stay on the pitch, and I think he should have gone off straight away because the second time it was a real hamstring [injury].
"I think now for him it will be tough to come back before the end of the season. It is usually 21 days [minimum].
"He should be OK to play for England, but I don't think he will be ready to play for us."
Source: PA
Source: PA