MIAMI — Marcus Smart was holding his shirt in his hand as he walked off the court following the Celtics’ shootaround Sunday morning. As he passed the team’s star forward, Jayson Tatum, who was completing a workout by himself, Smart playfully waved the shirt in Tatum’s face to block his view of the rim.
“Go ahead and shoot it,” Smart said, smiling. “Don’t be scared.”
About nine hours later, the Celtics took the court for Game 3 of the Eastern Conference finals against the Heat, and they soon looked shell-shocked. Three-pointers sunk to the floor without touching the rim. Miami center Bam Adebayo soared in for slams that were more suited for a dunk contest than a game of this magnitude. The Celtics’ stars looked like role players, and the eighth-seeded Heat’s role players looked like stars.
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It was a demolition, and as it snowballed, the Celtics appeared to have no desire to do anything about it.
Heat 128, Celtics 102 will go down as one of the more disheartening nights that this talented and big-dreaming core has ever stomached.
When this series began, Boston was the heavy favorite to win the NBA title. The path was navigable and visible.
Now, suddenly and startlingly, the Celtics find themselves on the brink of elimination, a possibility that would have been unfathomable just one week ago.
Teams that fall behind 3-0 in a best-of-seven series are 0-149 all-time.
“We could point fingers,” Celtics forward Jaylen Brown said, “but in reality, it was just embarrassing.”
The Celtics’ press conference room that has been constructed for this event in the bowels of the Kaseya Center is really just a few black curtains that surround a makeshift stage, and it sits about 30 feet from the tunnel that leads to the court.
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As coach Joe Mazzulla took questions about what had gone so horribly wrong, he had to speak up several times because of the roars coming from Heat revelers nearby. Not that anything he said was particularly revealing or encouraging.
Mazzulla took full blame for the Celtics’ puzzling lack of effort. In six of his first seven answers, he said that he had simply not gotten his team ready to play, and that he must do better. He repeated the maxim several more times later.
“I have to have the game plan ready for us to be physical and to execute,” he said, “and it’s important that we stick together.”
A loss to the Heat in this series would be one thing, but if the Celtics are punted into summer in a humiliating sweep, it will invite understandable and valid questions about what was once viewed as a core capable of challenging for NBA titles for years to come.
The Celtics still believe they have a chance, here and now, but after this loss even those affirmations were lacking their usual confidence. Brown was asked if he thinks this could be the end.
“I don’t think so. I would hope not,” he said. “That’s not what I’m about. I’m going to fight until the end.”
Tatum was asked if he was still confident.
“Yeah, we’d better be, right?” he said. “But we know it’s tough.”
Al Horford, the sturdy veteran who passes along wisdom wherever it fits, was probably the most optimistic.
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“We’re not out yet,” he said. “It is 3-0. I know what it looks like. I think I saw the other day, 0-149 or something like that. So, we’re not out yet. We’re still kicking.”
He said the team just needs to stick together. He said that’s been the key to their success. Then even he allowed some doubt to creep in.
“Right now,” he said, “it’s not looking so great. But, that’s my message.”
The Celtics’ confidence has been dented. But in the other locker room, it is soaring.
Miami entered the postseason in the play-in tournament, then lost its first game there, to the Hawks. Since then, it has been near-perfect. It toppled the Bulls to secure the final playoff spot, stunned the top-seeded Bucks in five games and then upset the fifth-seeded Knicks in six.
But its dominance in this series has been the most surprising and impressive of them all. The Heat continue to follow the unshakeable confidence and swagger of their leader, Jimmy Butler. He has made them believe they can do anything, and at this point, it’s understandable why they might feel that way.
On Sunday, Butler was fine but not dominant, and three former undrafted free agents pummeled a Celtics starting lineup that includes four former lottery picks.
Gabe Vincent (29 points), Caleb Martin (22), and Duncan Robinson (18) were the three leading scorers for the Heat, who shot 56.8 percent from the field and 54.3 percent from the 3-point line.
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Last season’s Celtics run to the Finals was constructed with defense. This group eventually found itself at that end of the floor, but it took some time. After Sunday’s loss, though, Mazzulla acknowledged there had been a setback.
“I think some of that defensive identity has been lost, and we have to get that back,” he said. “And that’s where part of that is on me to make sure we get that back.”
It remains to be seen whether there is time.
On offense, Tatum (14 points) and Brown (12) wilted, combining to go 12 for 35 from the field and 1 for 14 from the 3-point line, with six turnovers. Boston was 11 for 42 from beyond the arc as a team.
Miami led by as many as 22 points in the first half, but the Celtics pulled within 61-46 at the break, a manageable deficit in today’s NBA. Marcus Smart started the third quarter by soaring in for an offensive rebound and scoring as he was fouled, slicing the deficit to 12.
But that is where comeback hopes vanished. The Heat rolled off a 13-2 run that ended only because Miami received a technical foul for taunting as it rolled up the score. The game was never close again.
“They were together, they were physical, they set the tone, and we didn’t match the energy,” Brown said. “It was a complete letdown, to be frank.”
More Celtics coverage
- Dan Shaughnessy: Game 3 was a disgraceful performance by the once-proud Celtics. Even though it’s not over, don’t you want it to be?
- Instant analysis: Celtics quit in horrendous loss, and will now face their biggest challenge yet
- Celtics are on brink of elimination after epic 128-102 Game 3 loss to Miami | How it happened
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.