The team made the announcement during Sunday's game against the Miami Heat. The Celtics had lost six in a row heading into the nationally televised game despite back-to-back triple-doubles by the point guard who had been elected to start for the East in the Feb. 17 All-Star game in Houston.
Rondo injured his knee in Friday night's double-overtime loss in Atlanta, a game in which the Celtics blew a 27-point lead. He reported to the TD Garden on Sunday for a pregame shootaround but was taken to a hospital after complaining of knee pain.
The initial report was a hyperextended knee but tests showed the ACL tear, an injury that usually takes up to a year for recovery. Surgery had not been scheduled as of Sunday afternoon, said team spokesman Brian Olive, who confirmed that Rondo's season was over.
Rondo was averaging career highs of 13.7 points and 5.6 rebounds along with 11.1 assists per game this season. Still, the Celtics struggled to remain at .500, entering the day with a 20-23 record to hold the eighth and final spot in the Eastern Conference playoff race.