The England winger sustained an abdominal muscle strain which forced him out of the line-up, being replaced by German youngster Serge Gnabry, with Wenger admitting it is a bit of an unknown problem.
"He had an abdominal muscle problem," said Wenger.
"Is it a question of days? I hope so. He had a little problem before the Marseille game (on Wednesday) and it became a bit worse.
"It is a bit mysterious because we don't really know what it is."