Arsene Wenger
Wenger in the dark over Walcott's injury status. (Reuters)

Arsene Wenger has admitted he is not sure how long Theo Walcott will be unavailable for after a mysterious muscle problem ruled him out of Arsenal's 3-1 win over Stoke City.

The England forward was scheduled to start in Sunday's game at the Emirates Stadium but a knock he suffered before last week's 2-1 victory over Marseille in the Champions League worsened, forcing Wenger to replace him with 18-year-old Serge Gnabry.

The Arsenal boss has since admitted he is unsure of the extent of Walcott's injury, or even what the nature of the knock is, suggesting even the club's medical staff are temporarily unsure of how long it will take the 24-year-old to recover.

"He had an abdominal muscle problem, I don't really know what it is," Wenger told Sky Sports.

"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 and it then became a bit worse.

"It is a bit mysterious, because we don't really know what it is."

Arsenal showed no signs of struggling without Walcott as they sealed a comfortable win over Mark Hughes' side, a win that lifted them to the summit of the Premier League ahead of arch rivals Tottenham on goal difference.

Next up for the Gunners is a trip to the Hawthorns for a Capital One Cup clash with West Brom, before a trip to the Liberty Stadium next Saturday for a league game with Swansea City.

Given Arsenal's shortage of options up front, Wenger may once again call on Gnabry to fill in for Walcott should the England international be unavailable for those games.

After supplying three more goals for his teammates against Stoke, Mesult Ozil's quick adaption to the Premier League shows no signs of abating.

His fellow summer signing Mathieu Flamini has backed the Germany international to "be the difference" for the Gunners as they seek to end their eight year hunt for silverware.

"We knew Mesut was a big player - he proved it at Madrid," Flamini said. "For us, it is a great boost because he is a great player and knows how to make the difference.

"He has a great spirit, he is working hard for the team and can make the difference at any moment."