It is the latest in a series of injuries for the star, who underwent surgery in March for an ankle problem that kept him out of action for six months.
Fitness problems largely dominated his six seasons at PSG, which saw him sign for a world-record $234 million (222 million euros) in 2017.
He signed with Al Hilal in August for a reported $95 million.
Knee injuries are the most common and serious in football.
The CBF said Neymar’s injury was to his anterior cruciate ligament, one of the worst injuries ever suffered.
Surgery to correct the condition often requires players to be sidelined from action for months.