Antonio Rudiger is set to be out for Chelsea’s remaining four or five games after suffering a meniscus tear in his left knee and is being operated on in Rome on Tuesday, Goal understands.
The 26-year-old was taken off against Manchester United in Sunday's 1-1 draw at Old Trafford after suffering his second knee injury in the space of a fortnight.
The Germany international’s loss is a big blow for Maurizio Sarri as he has become a first-choice centre-back this season, starting 42 games in all competitions.
Rudiger's absence means Andreas Christensen is likely to step into the starting line-up to partner David Luiz for the remaining games of the campaign, with Gary Cahill taking up a place on the substitutes' bench.
The Blues can qualify for next season’s elite European competition by winning the second-tier Europa League or by securing a top-four Premier League finish.
The recent draw at Old Trafford was a boost to the Blues’ aims as they remained in fourth place and made it harder for Ole Gunnar Solskjaer’s side to overtake them in the process.
Rudiger joins Callum Hudson-Odoi on the injured list, with the England international out for the season after tearing his Achilles. Chelsea have no other injury complaints, with Willian likely to return on Thursday against Frankfurt after being taken off with a knock after a heavy challenge from Marcos Rojo in Sunday’s clash with United.
Speaking after the United game, Sarri felt Rojo's booking for his challenge on Willian was insufficient punishment.
"Were you happy? Willian was not so happy," Sarri added after the Brazil international hurt his ankle in the tackle.
"The card was an orange card, between yellow and red. We have to accept everything from the referee, it is not easy for the referee.
"My opinion - the yellow card was not enough, of course."