Barcelona expect Ousmane Dembele to return at the beginning of January following a successful operation on his hamstring.
However, after surgery in Finland, Barca now hope he will be back for close to the start of 2018.
In a statement released on Tuesday, the club said: "First-team player Ousmane Dembele has been operated on successfully on the rupture to the tendon of the femoral biceps of his left thigh by Dr. Sakari Orava and under the supervision of Dr. Ricard Pruna.
"The expected period of absence is three and a half months."
Barca president Josep Maria Bartomeu had expressed his optimism that their lower estimate on Dembele's length of absence would prove correct.
"I have spoken with him, he's affected, but that's normal," he told TVE. "He told me he knows everything will be okay, though, and that he's in good hands. By Wednesday or Thursday, he will be back in Barcelona.
"He could return in January, I hope so because that would represent a nice present for all Barcelona fans on Three Kings Day [January 6]."