Arsenal midfielder Abou Diaby has been ruled out for eight to nine months with a knee ligament injury.
Diaby, 26, missed most of last season with ankle and calf injuries and has made just 92 league starts for the Gunners since January 2006.
A club statement said: "Abou Diaby injured his left knee in training on Wednesday and he has torn his anterior cruciate ligament.
"Abou will undergo surgery to repair the injury in the near future."
Diaby's latest injury setback leaves Arsene Wenger without several key players.
Jack Wilshere is sidelined for two weeks and Theo Walcott will miss Saturday's visit of Reading after he suffered a muscle tear on England duty.