Ansu Fati sustained a knee injury during Barcelona's win over Real Betis, the club announced on Saturday.
Fati was substituted at half-time for Lionel Messi, with Barca later confirming in a statement: "Tests carried out this afternoon have shown that Ansu Fati has an internal meniscus tear in his left knee. The treatment to be followed will be determined in the coming days."
Any prolonged absence will be a serious blow for Barcelona, given the 18-year-old Fati has emerged as one of the team's most exciting players this season.
Fati has already scored six goals this season, including his first for Spain in September, which made him the country's youngest ever goalscorer.
He also became the youngest scorer in a Clasico when he scored against Real Madrid two weeks ago at Camp Nou.
Fati will withdraw from the Spain squad ahead of Nations League games against Switzerland and Germany later this month.