Arsene Wenger has revealed Jack Wilshere will be out of action for Arsenal for "a while" after suffering an ankle injury on Saturday.
The England international has been plagued by ankle problems during his career, and picked up a fresh complaint at the weekend after being sandwiched in a heavy challenge between Marouane Fellaini and Paddy McNair during the 2-1 defeat to Manchester United.
Ahead of Arsenal's UEFA Champions League clash with Borussia Dortmund, Arsenal manager Wenger was unable to say exactly how long the 22-year-old would be out for.
"I can't give you any timescale there," said Wenger. "He'll be out for a while.
"He has to see a specialist next week to see how long."
Wenger also revealed that Theo Walcott will miss the meeting with Dortmund due to a groin injury, while goalkeeper Wojciech Szczesny will also be absent after he hurt his hip in the build-up to United's first goal.
However, defender Laurent Koscielny is back in the squad after an Achilles problem, and Wenger is in a positive frame of mind ahead of Wednesday's game at the Emirates Stadium.
"It's an important game for us and we have a good dynamic in the team which gives us a good chance against Dortmund," he said.
"I see similarities between Arsenal and Dortmund. They have had many injuries at the start of the season too.
"I believe in what I do and I believe in my players, their quality and their spirit."
Arsenal require a point against Group D leaders Dortmund to be sure of qualifying for the last 16.