Antonio Rudiger will miss the rest of Chelsea's season due to a knee injury.
The Germany defender is now due to have an operation in Italy, having limped out of the Blues' 1-1 Premier League draw at Manchester United on Sunday.
Rudiger's loss, confirmed by the club on Tuesday, will prove a blow to Chelsea's bid for both a top-four Premier League finish and Europa League glory.
Fourth-placed Chelsea sit two points ahead of Arsenal with two league games to play, with the London rivals going head-to-head for the final Champions League qualification berth.
The Blues will face Eintracht Frankfurt in Thursday's Europa League semi-final first leg before hosting Watford on Sunday in their final home league game of the campaign. Maurizio Sarri's men will take on Eintracht in the return leg at Stamford Bridge on Thursday week, before closing their domestic season with a trip to Leicester.
But now they must do so without key centre back Rudiger, with his absence likely to be covered by Andreas Christensen.
Antonio Rudiger compared Maurizio Sarri's methods as Chelsea head coach to a school teacher.
The 60-year-old Italian's future as Blues boss is under scrutiny following humiliating losses at Bournemouth and Manchester City - the latter 6-0 defeat Chelsea's worst in 28 years - and faces a trying period.
Supporters' scepticism was voiced with an expletive-laden critique of 'Sarri-ball' - the possession-based style Sarri favours - in Monday's FA Cup loss to Manchester United.
And Sarri bemoaned a lack of time to mastermind an upturn in fortunes, with Thursday's visit of Malmo in the Europa League followed by City in Sunday's Carabao Cup final and Tottenham in the Premier League on February 27.
Sarri has introduced training on a match day, with Germany defender Rudiger downplaying the physical effects of the additional session, while admitting to the psychological challenge.
Rudiger said: "It's not like we are doing box to box runs. It's not physical. It's just the tactical things he likes to do.
"It's the same if you sit in the school the whole time listening to someone. Of course, sometimes, it gets mental, but at the end of the day you have to do your job."
Rudiger insists Chelsea cannot look beyond Malmo - the Blues hold a 2-1 first-leg advantage in their last-32 tie - but admits Sarri's Chelsea face a day of reckoning at Wembley on Sunday.
The defender said: "First of all, let's speak about the game tomorrow because it's important for us to stay in this competition and fight to win it.
"Then, on Sunday, a big game against City at Wembley. That will show the truth. That will show our way this season.
"The truth in terms of keeping up with opponents like City, and also Liverpool and Tottenham.
"You always play for your manager, but you want results. You want to win. No player on earth likes to have the kind of run we've had."