The Englishman, 24, had been thriving in manager Arsene Wenger's selection since returning to the first-team from an abdominal injury that required surgery.
Reintroduced to the senior squad in November, 2013, Walcott went on to feature in every match since (aside from the final Champions League game versus Napoli), recording three assists and five goals and it appeared likely that he could play a starring role in Arsenal's push for honours this season.
Walcott played most recently against Tottenham Hotspur in the heated North London derby that occurred in the the third round of the FA Cup at the Emirates Stadium but, despite his strong display, was brought off late in the game on a stretcher where it has since been confirmed that he ruptured the anterior cruciate ligament in his knee and will be out of action for six months - meaning he misses the entire season for both club and country.
A statement on the club's official website read: 'Theo will have surgery in London in the near future and is expected to be out for at least six months, therefore missing the remainder of the season and the World Cup in Brazil in the summer.'
Bendtner and Ramsey both took to Twitter to express their sorrow for the setback suffered by their team-mate.
Gutted for Theo, he'll come back stronger after this. Been fantastic for us so far this season. Have a speedy recovery mate— Aaron Ramsey (@aaronramsey) January 6, 2014
I am so sorry to hear about theos injury. He will be a massiv lost for the rest of the season. I wish him a speedy recovery and best of luck— Nicklas Bendtner (@bendtnerb52) January 6, 2014
image: © wonker