Barcelona's Ousmane Dembele (centre) will be out for four months
Dembele suffered a thigh injury in the 3-1 win over Getafe on Saturday.
The 20-year-old lasted just 29 minutes before limping off the field, to be replaced by Gerard Deulofeu.
Dembele cost Barca just under £100million this summer when he arrived from Borussia Dortmund to replace Neymar.
His injury comes as a major blow for manager Ernesto Valverde, who now faces a selection dilemma until the New Year.
But Bartomeu insists the club are confident Dembele will make a full recovery.
EPA
Ousmane Dembele went down in the 3-1 win over Getafe
"Unfortunately, Dembele is injured right now but will return,” he told Spanish TV station RTVE.
“Tomorrow he will be in Finland to see a specialist for this type of injury and when he comes back he will contribute.
“The doctor’s forecast estimates four months (out).
"I spoke with Dembele and he is a bit affected, because he was excited (about his start at Barcelona).
“But at the same time he trusts the Barca doctors.
“He's optimistic and when he comes back on Wednesday afternoon or Thursday he'll do it with a smile."
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Dembele is yet to score for Barca following his big-money move from Germany.
His withdrawal means Valverde will have to find an alternative in the Barcelona forward three that also includes Lionel Messi and Luis Suarez.
Speaking about replacing Dembele, Valverde said: "We have various players, Deulofeu, Aleix Vidal, Denis (Suarez), Andre (Gomes).
“We will look for alternatives to play in the first XI.