By Chris Harris
Theo Walcott's injury is worse than first feared and he is expected to be out for "three to four weeks".
Arsène Wenger broke the news to reporters shortly after leaving for Holland ahead of Tuesday night's Champions League clash with AZ Alkmaar.
Walcott limped off during the first half of Saturday's 3-1 win over Birmingham following a heavy challenge from Liam Ridgewell. It was the England international's first start of the campaign and he might have to wait a month for another one.
"The news is not good," confirmed Wenger. "We are without Theo Walcott for longer than expected.
"We thought it would be just 48 hours but it is a medial knee ligament, which is what I feared after the game. He will be out for three to four weeks."
Tomas Rosicky has also been ruled out of Tuesday's game with a knee problem but Wenger expects him to be fit in "a few days". However Gael Clichy has been passed fit after an ankle injury.
Nicklas Bendtner, Denilson, Eduardo and Johan Djourou are still out.
Arsenal squad to face AZ Alkmaar:
Robin van Persie