Arsenal manager Arsene Wenger says he is not sure how long forward Theo Walcott will be out for, or exactly what his injury is.
Walcott was all set to start in Sunday's 3-1 win over Stoke at the Emirates before picking up an injury in the warm-up.
And Wenger said: "He had an abdominal muscle problem, I don't really know what it is.
"Nobody can tell me whether it is just a question of days.
"He had a test before the game and I decided to play him but he had a little problem before the Marseille game (last week) and it then became a bit worse.
"It's a bit mysterious, because we don't really know what it is."
Arsenal are away to West Brom in the Capital One Cup on Wednesday before they travel to Swansea for a Premier League encounter on Saturday.