The 31-year-old was preparing for the new campaign alongside his teammates during training on Thursday when he was forced to abandon the session due to injury.
There had been a host of rumours surrounding the seriousness of the damage before Real Madrid confirmed that their number one stopper had ruptured the anterior cruciate ligament in his left knee.
Courtois will now undergo an operation before beginning his rehabilitation, but it is likely that he will be absent for the entire season due to the nature of the issue.
"Following tests carried out on our player Thibaut Courtois, he has been diagnosed with a rupture of the anterior cruciate ligament in his left knee. The player will undergo surgery in the coming days," read an official statement from the Spanish giants.
Widely regarded as one of the best goalkeepers in world football, Courtois has been with Real Madrid since August 2018, representing the club on 230 occasions in all competitions, keeping 91 clean sheets.
Last season, the Belgian was again a vital player for Carlo Ancelotti's side, featuring on 49 occasions in all competitions, keeping 17 clean sheets in the process.
Courtois's injury is set to open the door for Andriy Lunin to start Saturday's La Liga clash with Athletic Bilbao.
Real Madrid will move for a new goalkeeper before the end of the transfer window, though, with Chelsea's Kepa Arrizabalaga believed to be a loan option for the club.
David de Gea, who is a free agent following his release from Manchester United, is also allegedly being considered by senior figures at Real Madrid.
Sevilla's Bono is another to be linked with a switch to Bernabeu since the Courtois news was confirmed on Thursday afternoon.
Real Madrid will therefore be missing four players for their 2023-24 season opener against Athletic on Saturday, with Arda Guler, Ferland Mendy and Dani Ceballos also on the sidelines.
There remains concern when it comes to Guler's knee injury, and it is believed that the summer arrival could also be forced to undergo an operation in the coming weeks.