The striker is set to miss games against Tottenham Hotspur, Swansea City and Bayern Munich.
The striker lasted 14 minutes of the Gunners' Capital One Cup defeat to Sheffield Wednesday on Tuesday evening before a calf strain brought his evening to an abrupt halt.
It was an embarrassing moment for Arsene Wenger, as he and his staff had not ensured that Walcott had warmed up properly before replacing Alex Oxlade Chamberlain, who was forced off himself early on with a hamstring injury.
And that is looking like it will be a costly mistake, with Walcott set to miss the next three weeks with the ailment, ruling him out of the game with Swansea City this weekend as well as the north London derby with Tottenham and the vital trip to Bayern Munich in the Champions League, according to the Telegraph.
Losing Walcott for the game against Bayern will be a particular blow for Arsenal, with his pace having caused their defence numerous problems in the recent 2-0 win at the Emirates.
Gunners fans, and Arsene Wenger, must now hope that Olivier Giroud can step up and impress in his absence.