Chelsea goalkeeper Thibaut Courtois needs knee surgery and will be out for "quite a long time," manager Jose Mourinho said.
The 23-year-old Belgium international missed the champions' Premier League game at Everton on Sunday and will sit out the start of their Champions League group campaign.
Bosnian Asmir Begovic, signed from Stoke City this year, replaced Courtois, who sustained the injury in training.
"I cannot tell you much but I can tell you he needs surgery in his right knee and will be out for quite a long time," Mourinho told reporters.
"It is a big blow, it is not easy to have the best goalkeeper in the world injured, but it's easy to have one of the best in the world to play and we have Begovic to play," Mourinho added.
"I'm very sad for Thibaut and the team but we have a top keeper - Begovic will be in goal."
Chelsea, who sold long-time first-choice keeper Petr Cech to Arsenal in June, have made a disappointing start to the season, taking four points from their first four matches to fall eight points behind leaders Manchester City.
Courtois tore the meniscus of his right knee in training and is likely to be out until December following the operation, which is reported to have taken place in Spain.
Courtois remains in good spirits despite the injury and vowed to return to action as soon as possible.
"Thank you everyone for the support and the good wishes! I will work hard to recover as soon as I can!" Courtois Tweeted, alongside a picture of himself smiling in a hospital bed.
Courtois returned from international duty after playing in Belgium's 1-0 win over Cyprus last week, only to suffer the injury mid-week during a goalkeeper-specific training session.