Arsenal have announced Abou Diaby will be out of action for up to nine month after tearing his anterior cruciate ligament.
News of a potential problem emerged when manager Arsenal Wenger revealed Diaby was a major doubt for Saturday's Premier League clash against Reading, but it was unclear what the issue was.
It has since been revealed that he injured his knee in training on Wednesday with a subsequent scan confirming a cruciate ligament injury, which will require surgery.
"The Club can confirm that Abou Diaby injured his left knee in training on Wednesday and subsequent scans have confirmed that he has torn his anterior cruciate ligament," a statement on the club's website read.
"Following consultations with specialists, Abou will undergo surgery to repair the injury in the near future. Abou is expected to be out for around eight to nine months.
"The thoughts and best wishes from everyone at Arsenal are with Abou, and we all wish him a successful recovery."