Arsenal manager Arsene Wenger fears that Theo Walcott will be sidelined until mid-November with the knee injury he suffered during Saturday's 3-1 win against Birmingham City.
The England international's injury is worse than first feared and he is expected to be out for up to four weeks.
The Gunners boss said:"The news is not good. 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."
Walcott, who was starting his first Premier League match of the season, had to limp out of the game after being clattered by Liam Ridgewell.
Meanwhile, Tomas Rosicky has been ruled out of Tuesday's Champions League clash with AZ Alkmaar because of a knee problem while Gael Clichy has been passed fit after an ankle injury.
However, Nicklas Bendtner, Denilson, Eduardo and Johan Djourou remain on the sidelines.