Canada defender Kadeisha Buchanan has become Chelsea's sixth player to suffer an anterior cruciate ligament injury since December last year.
The centre-half was absent from Chelsea's midweek 2-1 Champions League victory over Celtic and their emphatic 2-0 victory over Manchester City in the Women's Super League on Saturday.
Manager Sonia Bompastor confirmed the injury following the victory over City that took Chelsea to the top of the WSL table.
"As I said, I'm really sad and I didn't want to talk about that situation before the game because maybe it would take all the focus from the game and I wanted my players to focus on the Man City game," Bompastor said.
"Everyone in the club is so devastated about the news. She was performing really well. She is an important player for us, but now this is the situation. We are going to support her as much as we can -- players, staff and the club. She's strong and she's OK."
Buchanan follows Sam Kerr, Aniek Nouwen and Mia Fishel, who suffered the injury last season, while Sophie Ingle and youngster Jorja Fox both tore their ACLs during preseason.
Buchanan is ruled out for the majority of the season, leaving Chelsea dangerously thin in the centre-back position.
Niamh Charles (shoulder) and Lauren James (ankle) are expected to return in the coming weeks.
