Diaby faces a lay-off of up to nine months after tearing the anterior cruciate ligament in his left knee.
The 26-year-old France international faces an operation on the injury, which he suffered in training on Wednesday.
A club statement confirmed: "Abou Diaby injured his left knee in training on Wednesday and subsequent scans have confirmed that he has torn his anterior cruciate ligament.
"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."