Walcott was injured in a challenge with Liam Ridgewell
Arsenal's Theo Walcott has been ruled out for "three to four weeks" after damaging knee ligaments in his side's victory over Birmingham on Saturday.
The 20-year-old winger, who missed the start of the season with a back injury, was substituted just 33 minutes into his first Arsenal start of the season.
"The news is not good," said Gunners boss Arsene Wenger.
"We thought it would be just 48 hours but it is a medial knee ligament, which is what I feared after the game."
Walcott returned to action for his club as a substitute against Blackburn on 4 October and featured in England Under-21's 6-3 win over Macedonia five days later.
He made a bright start against Birmingham at the Emirates Stadium but required treatment following a robust challenge by Liam Ridgewell and, despite returning to the field, was soon forced to make way.
Walcott will play no part in Arsenal's Champions League fixture away to AZ Alkmaar on Tuesday and could miss games against West Ham, Liverpool in the Carling Cup, Tottenham, Alkmaar in the return fixture and Wolves.
The winger may also struggle to be fit for England's friendly against Brazil in Doha on 14 November.
"The fact that he says he felt a little unstable when he was running is not good news," Wenger said after Arsenal's 3-1 win against Birmingham.
"We don't know yet how much of his pain is down to the kick and how much is down to the fact he twisted his ligament."
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