Arsenal will be without Theo Walcott for up to a month because of a medial knee ligament injury. Walcott, making his first start of the season, had to be taken off during the first half of Saturday's 3-1 win over Birmingham following a challenge from Liam Ridgewell. "The news is not good," said Wenger ahead of Tuesday's Champions League meeting with AZ Alkmaar. "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." Czech midfielder Tomas Rosicky is also unavailable for the AZ match, but full-back Gael Clichy has shaken off an ankle problem which kept him out of the Premier League win over Birmingham.