Lamine Yamal was held out of Barcelona's 1-0 LaLiga loss to Real Sociedad on Sunday with a "severe contusion" in his ankle the club said.
The 17-year-old picked up the problem in the 5-2 Champions League win over Red Star Belgrade in midweek.
His involvement with Spain in upcoming UEFA Nations League games against Denmark and Switzerland on Nov. 15 and Nov. 18 respectively is also in doubt.
"Yamal sustained a severe contusion in Wednesday's game against Red Star Belgrade," Barça said in a statement.
"Over the past few days, he has undergone treatment, but the discomfort has yet to subside. His recovery will determine his availability."
After Sunday's game in San Sebastián, Barça are not in action again until Nov. 23, when they travel to Celta Vigo.
Yamal has played a key role in helping Barça to the top of the LaLiga table so far this season, scoring six goals and setting up eight more in 16 appearances in all competitions.
Barcelona, who have lost both games in which Yamal did not start, are still in first place in LaLiga despite the loss, with Hansi Flick's side sitting on 33 points from 13 games, six ahead of second-placed Real Madrid.
"I think it would be an easy answer to say 'OK, we lost because Lamine wasn't there,'" Flick said after the match. "Of course a player like this quality Lamine has, every team will miss it. But at the end, we started good. I think the goal, what I can say it was wrong decision, absolutely, but we have to accept it.
"After the La Real goal we suffered a little bit. I think today was not our day, also in the offence we had not the clear chances to make goals. We made wrong decisions in the last third. We have to accept it today."
