Chelsea centre back Thiago Silva has damaged the ligaments in his knee and will undergo rehabilitation, the Premier League club said on Tuesday.
The Brazil international picked up the injury in the first half of their 2-0 defeat at London rivals Tottenham Hotspur in the league on Sunday.
The 38-year-old underwent further assessment and a scan on his return to the training centre on Monday, the club said.
"Scan results from those assessments have confirmed damage to Thiago's knee ligaments and he will now work closely with the club's medical department during his rehabilitation to return to action as soon as possible," Chelsea said in a statement.
The defeat at Spurs extended Chelsea's miserable run under manager Graham Potter, with the team having won only two of their last 15 games in all competitions.
Chelsea are 10th in the league with 31 points after 24 games, 14 points outside of the top four. They host Leeds United on Saturday.
West Ham suffer Fabianski injury blow
West Ham United will be without goalkeeper Lukasz Fabianski for a lengthy spell after he suffered a fractured cheekbone in Saturday's Premier League win over Nottingham Forest.
"He's got a fractured cheekbone and eye-socket," manager David Moyes, speaking ahead of Wednesday's FA Cup fifth-round tie at Manchester United, told a news conference.
"We don't quite know whether it'll be operated on or healing itself, or a time-span at the moment."
Fabianski's injury is a blow to West Ham, who are two points above the relegation zone with 14 league games left.
The 37-year-old has started every game in the Premier League this season.
West Ham will now have to turn to second-choice keeper Alphonse Areola, who came on against Forest after Fabianski was injured.
"We've had the surgeons report back but I think there's a decision to make as to whether it's surgery. It's huge for us and huge for the goalkeeper as well.
"The stage of the career for him as well."
Moyes confirmed that West Ham will also be without right back Vladimir Coufal for the trip to Old Trafford because of a heel injury.
Striker Danny Ings, who scored his first two goals for the Hammers against Forest, is also not available as he is cup-tied after appearing previously in the competition for Aston Villa.
