Arsenal confirmed on their website that Walcott is likely to be out for at least six months having sustained the injury in Saturday's 2-0 FA Cup win over Tottenham Hotspur at the weekend.
He was taken off on a stretcher after appearing to jar his knee during the closing stages although the injury has turned out to be more severe. The 24-year-old did not appear to be in severe pain and even signalled "2-0" at Spurs fans, as missiles rained down on him.
Walcott learned yesterday he would face no action for his hand gesture, but bad news followed good.
"Theo will have surgery in London in the near future and is expected to be out for at least six months," read an Arsenal statement.