News of the issue came via Kristof Terreur of Het Laatste Nieuws, although it was noted the injury is not a serious one:
Chelsea later confirmed their XI for the clash on the south coast on their Twitter account, with Willy Caballero taking the Belgian's place:
It's a big blow for the Blues in the short-term, as they will also be without the suspended duo of Pedro and Alvaro Morata at the Amex Stadium.
Courtois has been a key man in Antonio Conte's Chelsea side, offering a commanding presence at the base of the team.
Blues supporters would have been delighted to see him return to form under the guidance of the Italian coach. Courtois had a difficult 2015-16 season, suffering with an injury and never really finding his best form in a tough term for Chelsea.
However, since the start of the 2016-17 season, Courtois has been back at his imperious best. The Belgian shone for Atletico Madrid during three campaigns on loan and was a pillar of consistency when Chelsea won the league in his first campaign as a starter in 2014-15.
He's undoubtedly one of the best stoppers in the world at this point in time. And while the Blues have an able deputy in former Manchester City 'keeper Caballero, Chelsea will miss the 25-year-old.
Courtois is authoritative when coming off his line, is improving in his distribution and uses his rangy build to stunning effect at times, getting his hands to shots that he has no right to save.
Conte's side have looked much more secure with Courtois underpinning the team, and he'll be missed against a battling Brighton side.