Rondo injured the knee on Friday night while playing the Atlanta Hawks, who came back from a 27-point deficit to win in double overtime.
The injury was originally called a hyperextension, but Rondo felt discomfort in the knee during the Celtics' morning shoot-around Sunday. He was taken to a hospital, where an MRI exam revealed the torn ligament.
Rondo had been having a banner season, registering triple-doubles in back-to-back games last week against the Cleveland Cavaliers and New York Knicks while leading the NBA with 11.1 assists a game. Despite his efforts, the Celtics had fallen below .500 and into eighth place in the Eastern Conference during their losing streak and are now 21-23 this season after the win over the Heat.
Rondo had been averaging career highs in points (13.7) and rebounds (5.6) as well this season, when he was shooting 48.4% from the field.