Coutinho, 28, will have an operation this week, after which the club will confirm exactly how long he will be out of action.
The Brazil international has only recently come back from a hamstring problem and came on as a substitute as Barca dropped points against Eibar at Camp Nou.
He was unable to finish the game after hurting himself when twisting to play a pass, with Barca confirming on Wednesday he has an "injury to the lateral meniscus in his left knee."
A statement from the Catalan club said: "Treatment will necessitate arthroscopic surgery which will take place in the coming days."
Coutinho returned to Barca this season following a one-year loan at Bayern Munich, with coach Ronald Koeman promising him an important role.
He returned to action in November but has been in and out of the team since and is now set for another lengthy spell on the sidelines.
In total, he has made 14 appearances under Koeman, scoring three goals and creating two more.
Sergi Roberto, who can play at right-back or in midfield, is also not too far away from a return to first-team football.
Barca's draw with Eibar represented the eighth time this season they have dropped points in La Liga in 15 games. They are currently sixth in the table.