The 20-year-old, who joined the Catalan giants from Borussia Dortmund during the summer, sustained a thigh injury in Saturday's 2-1 victory over Getafe in La Liga.
The France international was signed as a direct replacement for Neymar, who joined Paris Saint-Germain during the off-season, but the club have now confirmed he requires an operation on the injury and faces up to four months on the sidelines.
A club statement read: "The tests have come back following first team player Ousmane Dembélé has ruptured the tendon in his femoral bicep in his left thigh.
"The player will be operated on by Dr. Sakari Orava in Finland this week. He is expected to be out for between three and a half months and four months."
Dembele is now likely to miss the rest of Barcelona's group stage campaign in the Champions League, while he could also be absent for December's El Clasico against Real Madrid at the Bernabeu.