Wenger's joy was tempered by a knee injury to Walcott. The England winger limped off in injury time after pulling up on the halfway line with no-one around him.
Walcott is now a doubt for Wednesday's Champions League clash against Olympiacos and next Sunday's north London derby at Tottenham.
"I don't really know what happened with Theo," Wenger said.
"I thought it was a classic hamstring injury but it is not.
"He felt a sharp pain in his knee. I don't know what the diagnosis will be but it was completely surprising. I don't know if he will have scan tonight. I will have to speak to the medical staff."