Courtois suffered the injury on Thursday and he is set to undergo surgery to ascertain the seriousness of the issue.
Reports suggest the Belgium goalkeeper could be out until April at least and Real Madrid will have to dip into the transfer market for a new shot stopper between the sticks.
A short statement on Real Madrid's website read: "Following tests carried out on Thibaut Courtois, the player has been diagnosed with a torn anterior cruciate ligament in his left knee.
"The player will undergo surgery in the coming days."
Real Madrid will begin their La Liga campaign against Athletic Bilbao on Saturday.
Courtois has made 230 appearances for Real Madrid since joining from Chelsea in the summer of 2018.
The 31-year-old has won two La Liga titles, one UEFA Champions League, one Copa del Rey, one UEFA Super Cup and one FIFA Club World Cup.
Real Madrid could turn to David De Gea, who is a free agent after leaving Manchester United at the end of last season.
