Real Madrid suffered a huge blow on the eve of the new La Liga season with goalkeeper Thibaut Courtois rupturing the anterior cruciate ligament in his left knee during training on Thursday.

The 31-year-old Belgium international faces a lengthy spell on the sidelines and will undergo surgery in the coming days, his club said.

"After the tests carried out on our player Thibaut Courtois, he has been diagnosed with a torn anterior cruciate ligament in his left knee," Madrid said in a statement.

The club did not specify how long Courtois will be out but reports in Spain suggest he will miss most of the season, if not all of it.

Courtois suffered the injury during a morning training session and left in tears, according to Cadena Cope radio.

He will be replaced in the team by Ukraine's Andriy Lunin, with Madrid due to play Athletic Bilbao on Saturday in their first game of the season.

Courtois joined Madrid from Chelsea after the 2018 World Cup, helping the club to a pair of La Liga crowns and last year's Champions League title.