The 18-year-old Spain international was substituted at half-time in Saturday's 5-2 win over Real Betis, with Barca later confirming the damage sustained to his left knee.
"The first team player Ansu Fati has been operated on successfully by Dr. Ramon Cugat for the injury to the internal meniscus of his left knee under the supervision of the Club's medical staff. The player will be out for approximately four months.
"Ansu Fati played the first half of Barça v Betis, and was very active. This season, the young Barça forward has played 10 games, 7 in the league and 3 in the Champions League. In total Ansu has played 596 minutes this season, in which he has managed to score five goals and provide two assists."