Outgoing Arsenal manager Arsene Wenger revealed this in his press conference on Tuesday as he prepares for his penultimate Premier League game away to Leicester City on Wednesday.
Koscielny, 32, was carried off 12 minutes into Arsenal’s Europa League semi-final second leg 1-0 defeat by Atletico Madrid last Thursday.
France manager Didier Deschamps had already confirmed the centre-back would miss this summer’s World Cup.
“Koscielny is devastated. He has had surgery and is out for six months. You will not see him until December at best,” Wenger said.
“He will miss the World Cup.
“People have always inflamed tendons but they don’t rupture but in his case it went. I don’t know why.
“You always try to be positive. Try and support people when they suffer.”