The injury-prone France midfielder has damaged his left anterior cruciate ligament and will undergo surgery before a long spell of rehabilitation.
The Gunners are already without Jack Wilshere, who is out with an ankle problem, although he should return before the end of the season.
“Abou Diaby injured his left knee in training on Wednesday and subsequent scans have confirmed that he has torn his anterior cruciate ligament," Arsenal said in a statement.
"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."
Diaby, 26, has suffered a broken ankle, a serious thigh tear and a multitude of niggling, recurring injuries since joining the North London club from Auxerre for £2 million in 2006.
His best season, the 2009-10 campaign, saw him play 29 Premier League matches but he only managed four games in 2011-12 and 10 this term.
The Paris-born midfielder has been capped 16 times by France but Auxerre’s decision to sell him on the cheap appears to have been vindicated by his failure to complete a full season without lengthy spells on the sidelines.