Barcelona have confirmed that Ousmane Dembele will be out for between three-and-a-half and four months after tests revealed he has ruptured a tendon in his thigh.
The Frenchman was forced to withdraw in the first half of his side's 2-1 win against Getafe with what at first appeared to be a hamstring pull.
But it has now been confirmed that the injury is far more serious, and could see Dembele miss the first half of the season.
The 20-year-old, who was making his first league start for his new club, will now fly out to Finland where he will undergo surgery.
A club statement after the game read: "Ousmane Dembele has a hamstring injury in his left leg, more tests are needed to find out the extent of the injury."
Coach Ernesto Valverde echoed the statement, and admitted that losing the 20-year-old to injury so soon is "a shame".
"It's a muscle problem," he said. "We think it's a tear, but we will do more tests tomorrow.
"It's a shame because he was just getting up to scratch with the team's dynamic. We hope he won't be out for a long time."
Barcelona signed Dembele from Borussia Dortmund for an initial €105m, although it could rise to €145m with add-ons.
The youngster made his debut as a substitute in last weekend's 5-0 victory over Espanyol, providing an assist for Luis Suarez.
He then made his first start in Tuesday night's Champions League opener against Serie A champions Juventus.