Barcelona confirmed it has detected an issue in the soleus muscle in Frenkie de Jong's calf after missing the draw with Sevilla.
The Netherlands international had to miss the trip to the Ramon Sanchez Pizjuan, where Barca was frustrated in a goalless stalemate.
It was confirmed a day earlier that De Jong had been ruled out through injury, though the club did not reveal what the issue was.
After further medical tests, Barca was able to determine the former Ajax talent is suffering with a problem in his soleus muscle in the right calf.
However, the club has not commented on how long he could be out for, saying the "evolution of the injury will condition the player's availability."
De Jong is in his first season at Camp Nou and has made 38 appearances across all competitions.
Barca is next in action at home to Athletic Bilbao on Wednesday (AEST), but by then it could have surrendered its place atop of LaLiga.
The draw in Seville leaves Barca just three points ahead of Real Madrid, which has a game in hand.
Should Los Blancos win that match – away to Real Sociedad on Monday – they will go top of the league due to their superior head-to-head record against Barca.