Fati, 18, was taken off at half-time against Betis and Barca have since confirmed that "he has an internal meniscus tear in his left knee." Sources have told ESPN it is too early to tell how serious the injury is but in the best-case scenario the club are hopeful they will only be without Fati for between four and eight weeks.
Barca must wait for the swelling to subside in the coming days before deciding on a course of treatment for the Spain international. He could be out for up to three months if surgery is required.
Meanwhile, if he doesn't play again until the New Year, he could miss as many as 10 Barca games, including the trip to Atletico Madrid straight after the international break.
Fati picked up the injury after being fouled by Betis defender Aissa Mandi in an incident that drew a penalty. He played on for 15 minutes after the challenge -- despite needing lengthy treatment on the pitch at the time -- before finally being replaced by Lionel Messi.
Ansu has scored five goals in 10 appearances this season for Barcelona and has also registered two assists.
Without him, Barca went on to beat Real Betis 5-2 to end a four-game winless run in La Liga. Messi scored twice, with Ousmane Dembele, Antoine Griezmann -- who also missed a penalty -- and Pedri also on target.