Justin hurt his knee in Leicester's FA Cup win over Brighton and Hove Albion on Wednesday and was taken off on a stretcher.
Manager Brendan Rodgers confirmed the severity of the injury in Friday's pre-match media conference ahead of their Premier League meeting with Liverpool.
"Sadly the scan hasn't shown up so well, which is a massive blow for us," Rodgers said.
"He looks like he's done his ACL, which is a devastating blow for us because he's been incredible since he came into the team last year.
"He's played in many positions. Wherever I've asked him to play, whatever position, he's been a minimum eight out of 10 and developing as a top Premier League player."