Arsenal manager Arsene Wenger admits he isn't sure the extent of Theo Walcott's injury after the striker was forced to pull out of Sundays fixture with Stoke City in the warm-up.
The England international was named to start at The Emirates Stadium but an abdominal injury forced Wenger to play Serge Gnabry for only the second time in the Premier League.
However, The Gunners boss knows no more on the injury.
The 63-year-old told SkySports: "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 already without fellow attacking player Lukas Podolski therefore the north London club can ill afford any more absentees in that department.
The 36 cap international Walcott has become an integral part of the line-up since arriving in 2006, he has scored 40 goals from 185 games.
The remaining players ensured that the 24-year-old wasn't missed too much as they ran out 3-1 winners.
New signing Mesut Ozil picked up a hat-trick of assists while Aaron Ramsey, Per Mertesacker and Bacary Sagna got the goals.
That result means Wenger's side are top of the table after five games.
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