Barcelona have confirmed that Marc Bernal ruptured his anterior cruciate ligament in Tuesday's win over Rayo Vallecano and will undergo surgery in the coming days.
Barça have not confirmed how long Bernal is expected to be out but it is anticipated he will miss the majority of the season given the severity of the injury.
Bernal, 17, picked up the injury in the final minutes of the 2-1 win at Rayo when he twisted his knee going in for a tackle.
It became instantly clear how serious it was, and coach Hansi Flick said it made it difficult to celebrate what was Barça's third straight win at the start of the new LaLiga season.
"It's a sad victory because Marc Bernal is injured," Flick said in a news conference after the game.
"We win, it's OK, but when you see the dressing room nobody is happy.
"It's not good. He made a fantastic match -- 17-years-old, such a performance. It hurts."
Bernal has been one of the positives of Barça's bright start to the season, making his debut for the club in the opening day win over Valencia after impressing Flick in preseason.
Playing at the base of the midfield, the Spain youth international is one of three 17-year-olds to have started all three of the Blaugrana's league games so far, along with defender Pau Cubarsí and winger Lamine Yamal.
Meanwhile, young left-back Álex Valle has joined Celtic on a season long loan.
The 20-year-old extended his contract until 2026 ahead of joining the Scottish side, where Barça value the fact he will have the chance to play Champions League football.
Valle spent last season on loan at second division side at Levante and his highly rated at Barça, where he did preseason under Flick.
