LeBron James, Jordan Clarkson rip Celtics 121-99 on Paul Pierce retirement day

Celtics embarrassed by new-look Cavs as No. 34 heads to the rafters.

There wasn't much to smile about for the Big 3 and Doc Rivers once the game began Sunday. (Kevin Garnett photo)

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BOSTON — Leave it to LeBron James to ruin the vibe for Paul Pierce.

Powered by 24 points, 10 assists and eight rebounds from the Cleveland superstar, the new-look Cavaliers dismantled the Celtics, 121-99, Sunday afternoon before a national TV audience and courtside crowd that included Doc Rivers, Rajon Rondo and Kevin Garnett.

Call it the revenge of Game 7. That’s when Pierce beat James in a mano-a-mano battle at TD Garden to advance to the Eastern Finals.

One of the keys in the trade of Isaiah Thomas to the Lakers was Jordan Clarkson. He lit up the Celtics for 17 points on 7-of-11 shooting.

The Celtics have now trailed by at least 26 points in three of their last four games, losing all three.

Boston (40-18) have already lost the top seed in the East and could be in danger of watching a rejuvenated Cleveland team pass them in the standings if Sunday is any indication.

With electricity and adrenaline pumping through TD Garden at an NBA Finals fever pitch, the game more than met the level of anticipation.

With Brad Stevens telling reporters before the game that his team needed to attack the paint more, Kyrie Irving drove to the basket for a layup in the opening 10 seconds.

But Irving had much more in store in the opening minutes. With LeBron James guarding, Irving drove at him, crossed him over and then drained the fallaway jumper. The building erupted even before he raised his shot.

The Celtics led 32-31 after one quarter. It was all downhill from there. The Celtics played little defense and allowed the Cavs to outscore them 33-20 in the second quarter.

James had another classic moment of drama when he came up lame in the second quarter and was tended to by trainers on the Cleveland bench. He would return later in the half.

Things only got worse in the second half. Jaylen Brown missed his first six shots and the Celtics trailed 95-74 after three quarters.

The Celtics trailed by as many as 29 (three times) in the fourth quarter and never got close. The Celtics play the Clippers on Wednesday at TD Garden.