Fati suffered a serious meniscus injury in November, having scored five goals across 10 appearances for Barca across all competitions.
He had initially been forecast to return in March, though the 18-year-old hinted in a social media post at the end of that month he may be facing a longer spell on the sidelines.
Having failed to return to training throughout April, Fati has now gone under the knife yet again, this time for a revision arthroscopy in his left knee.
Barca confirmed in a statement on the club's official website that the operation had been undertaken by Dr. Jose Carlos Noronha in Porto.
The Blaugrana have offered no update on a timescale for Fati's return, though it will certainly rule the youngster out of Euro 2020, albeit his chances of making the rearranged tournament already seemed extremely remote.