"The tests performed on first team player Ousmane Dembele have determined he has ruptured the tendon of the hamstring in his left leg," Barcelona said in a statement on Sunday.
"The player will undergo surgical treatment by Dr. Sakari Orava this coming week in Finland. The expected time out will be between three-and-a-half and four months."
Dembele has made just three appearances for Barca since sealing his move from Borussia Dortmund last month for a fee that could rise to as much as 145 million euros based on performance-related bonuses.
However, he may have paid the price for missing much of pre-season as he went on strike from training at Dortmund to try and force through his dream move to the Camp Nou.
The 20-year-old is now set to miss the rest of Barca's Champions League group campaign and faces a race against time to take part in the first Clasico of the season against Real Madrid on December 23.