The 32-year-old France international underwent surgery on a torn Achilles tendon and had already been ruled out of this June and July's World Cup.
"Koscielny is of course devastated because he had surgery. That means he will out for six months," Arsenal manager Arsene Wenger told reporters on Tuesday.
"You will not see him now certainly until the end of December."
Wenger stands down at the end of the season.
The Frenchman leaves Arsenal after 22 years in charge, with media reports suggesting on Tuesday that the club will have a new manager in place before the World Cup.
"Koscielny is of course devastated because he had surgery. That means he will out for six months," Arsenal manager Arsene Wenger told reporters on Tuesday.
"You will not see him now certainly until the end of December."
Wenger stands down at the end of the season.
The Frenchman leaves Arsenal after 22 years in charge, with media reports suggesting on Tuesday that the club will have a new manager in place before the World Cup.