The 23-year-old Belgium international damaged his medial ligaments during training on Wednesday.
Manager Jose Mourinho confirmed the extent of the injury at his media briefing before Saturday’s trip to Everton.
“It is a big blow, it is not easy to have the best goalkeeper in the world injured,” said the Chelsea boss.
Former Stoke goalkeeper Asmir Begovic will replace Courtois, having already stood in for the Belgian when he was sent off against Swansea on the opening day of the season.
“I’m very sad for Thibaut and the team but we have a top keeper and Begovic will be in goal,” said Mourinho.
After spending three seasons on loan at Atletico Madrid, Courtois was made first-choice Chelsea keeper for the 2014-15 campaign, displacing Petr Cech.
Begovic was brought to Stamford Bridge following Cech’s switch to Arsenal in the summer.