Kurt Zouma will almost certainly miss the rest of the season and Euro 2016 with a knee injury which could have implications for John Terry's future at Chelsea.
France defender Zouma injured his anterior cruciate ligament in Sunday's 1-1 Barclays Premier League draw with Manchester United.
A statement on Chelsea's official website read: "Kurt will undergo surgery in the next 48 hours and is expected to be out for approximately six months."
Zouma landed awkwardly early in the second half of the drawn Premier League contest and was carried off on a stretcher. He vowed to return "stronger" on Monday.
The 21-year-old wrote on Twitter: "Scan today (Monday) has shown I injured my ACL. I will have surgery this week & will come back stronger. Thank you everyone for all the messages."
Zouma has started 16 of the last 19 Chelsea matches in all competitions, displacing Gary Cahill as first-choice alongside captain Terry.
Cahill, the England vice-captain, will welcome the opportunity to return to the starting XI, even if the circumstances are unfortunate for a team-mate.
Cahill will likely line up alongside captain Terry in Saturday's Premier League clash with Newcastle.
Terry last Sunday made his first appearance at Stamford Bridge since declaring Chelsea would not be extending his contract this summer.
The 35-year-old skipper thanked supporters following the draw with United, but said there had been no communication with the club over his future.
The Blues maintain a contract offer could yet be tabled to Terry, who is set to make his 699th Chelsea appearance against the Magpies.
And Terry will likely be first choice with Cahill for the rest of the campaign, giving him the opportunity to make a compelling argument that he should stay.
Spain full-back Cesar Azpilicueta told the London Evening Standard: "It's a situation between him and the club. I don't know what's happening.
"But since I arrived at Chelsea I have learnt a lot from him. I have played alongside him on the left for most of the time, even in training.
"For me, I see no difference in the way he's performing on the pitch, the way he's playing and he is giving everything for the team as always."