The Clippers, who have lost five of eight, played without Leonard after he was ruled out shortly before the game due to back spasms.
Leonard was listed as a starter before the game, but never took the floor during warmups. The team announced late in the first quarter that he had been ruled out.
He has missed seven previous games this season, including two for a mouth laceration, two while in the health and safety protocols and three for a bruised leg.
Leonard’s absence comes with Los Angeles on the tail end of a five-game road trip, which concludes Thursday at Washington. He was coming off a game in which he played 37 minutes in the Clippers’ loss at Milwaukee on Sunday.
In a game that featured 16 lead changes, the Celtics took a 91-89 edge into the fourth quarter. It grew to 111-100 on a layup by Tatum with 4:23 remaining.
A 3-pointer by Jackson and free throw by Patrick Beverley got Los Angeles within 113-109 at the 1:53 mark.
That was still the score when the Clippers got the ball back with 26 seconds left after Tatum was called for an offensive foul for pushing off on George.
George got the ensuing inbounds pass but missed a 3 off the back of the rim. Robert Williams was fouled and connected on one of his two free throws.
George hit a 3 on the Clippers’ next trip to cut it to 114-112. Daniel Theis was fouled, but calmly sank two free throws to help seal it.
The Celtics took advantage of not having to contend with Leonard on the defensive end and started the game 9 of 10 from the field to take an early 20-13 lead.
But George picked up the offensive slack for the Clippers, scoring 21 points in the opening 24 minutes to put Los Angeles in front 63-62 at the half.
Celtics coach Brad Stevens said Marcus Smart, who missed his 17th consecutive game with a left calf tear, is getting closer to a return. Smart worked out before the game, doing some shooting and footwork drills. With three practices planned between their first two games following the All-Star break, Stevens said it will give them a better idea of when he’ll be ready.