The forward lasted just six minutes of Brighton's 2-1 FA Cup third round win over Crystal Palace on Monday, just 24 hours after his 21st birthday, before needing to be substituted, with fans fearing the worst.
And, those concerns were justified as scans revealed the extent of the injury, with Brown returning to Stamford Bridge to begin the long road to recovery.
"It's one we didn't expect. It's really disappointing news for all concerned, but obviously more for Izzy than anyone else," manager Chris Hughton told Brighton's official website.
"He's a young lad who has made a really good impression at the club and in the second half of the season he would have played more games.
"It's still very fresh for everyone, but we are grateful to Chelsea for the loan, and to Izzy for the impact he has made during the first half of the season."