The 31-year-old Belgian international left the practice field in tears and an MRI scan confirmed the ACL tear two days ahead of Real's LaLiga season debut at Athletic Bilbao.
Goals by Karim Benzema and substitute Marco Asensio gave reigning champions Real control in the Spanish capital but with Chelsea down to 10 men for more than half an hour, Courtois said the mood was subdued in the dressing room.
Defender Militao finally prised open the visitors by heading home from a corner in the 76th minute for his first league goal of the season.
The Spanish giants lost the third match in the league this season and they are fourth in the standings with 17 points after 10 matches.
Courtois to get them out of trouble this season and they will be able to lean on him again in Saturday’s La Liga game against Cadiz after he overcame a small injury scare.
Real have 86 points after 37 games, seven more than deposed champions Barcelona.
Navas says the signing of Belgium keeper Thibaut Courtois has not affected his desire to remain at the triple European champions.
Chelsea goalkeeper Thibaut Courtois wants a move back to Spain for family reasons, his agent said, following reports linking him with a move to Real Madrid.
"Clearly we would always like to keep all the strongest players," said Sarri, who began in English but switched to his native Italian so he did not "say anything wrong".
Mourinho has switched to a back-three as well as used midfielder Ander Herrera to man-mark Eden Hazard in the two teams the last few encounters.
Chelsea goalkeeper Thibaut Courtois has withdrawn from the Belgium squad for Tuesday's friendly against Saudi Arabia due to a hamstring injury, the Belgian Football Association (RBFA) said on Sunday.
Five days after losing 3-0 at home to Bournemouth, Chelsea's defence of their league title hit a new low when they were beaten 4-1 at Watford.
Hazard and Courtois have been linked with a move to Spanish giant Real Madrid.
Eden Hazard’s father told press on Friday that his son rejected an offer to renew his contract with Chelsea, increasing chances of his transfer to Real Madrid.
Courtois shared Atletico Madrid dressing room with the Spanish striker, Diego Costa