Rondo sustained the injury during Friday's double-overtime loss to Atlanta.
It was originally diagnosed as a hyperextended right knee, but a magnetic resonance imaging exam revealed a torn anterior cruciate ligament, the Celtics said.
A date for surgery to repair the knee will be set soon.
Rondo is the NBA's assists leader, averaging 11.1 helpers per game, while posting a per-game scoring average of 13.7 points over 38 contests this season.
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