London - Chelsea's troubled start to their Premier League title defence suffered a fresh blow on Friday when it emerged that goalkeeper Thierry Courtois required surgery on a knee injury.
"Courtois was injured in a training session with the goalkeepers and I cannot tell you much, but I can tell you that he needs surgery in his right knee," Chelsea manager Jose Mourinho said.
He added: "As a consequence of that he will be out for quite a long time. I cannot be at this moment more specific about the injury or the time he will be out."
The English champions will first be without the Belgium international for Saturday's match at Everton.
Chelsea have picked up only four points from their opening four games and sit eight points behind leaders Manchester City.
Asmir Begovic will deputise for Courtois, who suffered the injury on Wednesday.
"In this moment we lose Courtois for a long, long time - I hope it's not long, long, just long," added Mourinho, who will have Jamal Blackman on the bench.
Oscar remains out with an undisclosed injury, but could return to contention for Wednesday's Champions League campaign opener at home to Maccabi Tel Aviv or the following Saturday against Arsenal.
"Almost there," said Mourinho on the Brazil playmaker.