The Arsenal midfielder will miss the remainder of the season, along with England's World Cup campaign, after he ruptured his anterior cruciate ligament.
Walcott was stretchered off during the club's 2-0 win over Tottenham in the FA Cup third round on Saturday and scans have revealed he has ruptured his anterior cruciate ligament.
The 24-year-old will undergo surgery to rectify the problem, which will sideline him for the remainder of the Premier League season and the World Cup in Brazil this summer.
Walcott courted controversy during Saturday's match as he gestured the scoreline to the Tottenham fans while being stretchered off.
The Football Association confirmed Monday that the winger would face no disciplinary action for the gesture, but now injury has brought a premature end to his campaign.
Walcott has made 21 appearances for Arsenal and England this season, scoring seven goals.