MIAMI — Joe Mazzulla’s roller-coaster season is at its lowest. And with the Boston Celtics now on the brink of elimination, the first-year coach is blaming himself.

The Celtics are in the sort of trouble that no team in NBA history has escaped, trailing Miami 3-0 in the Eastern Conference finals after a 128-102 Heat romp on Sunday night that might not have been even as close as the score would make it appear.

“I think the most important thing is just sticking together, and then I have to be better,” said Mazzulla, the NBA’s youngest coach at 34. “I’ve got to put them in better positions. I’ve got to get them ready to play. I have to have the game plan ready for us to be physical and to execute, and it’s important that we stick together.”

It has been a disaster of a series for the Celtics. They’re letting the Heat shoot 52% from the field, 48% from 3-point range — compared to a 29% effort from deep by the Celtics — and the dam might have broken in Game 3. After Games 1 and 2 were decided late in Boston, with the Heat finding a way both times, this one was never in doubt.

“I just didn’t have them ready to play,” Mazzulla said.

That’s quite an indictment, especially after how the last eight months or so have gone for Mazzulla. He wasn’t supposed to coach the Celtics this season, getting the job on an interim basis out of necessity once Ime Udoka was suspended. The regular season was one success after another; the Celtics removed the interim tag just past the season’s midpoint, he coached in the All-Star Game and finished third in the Coach of the Year balloting.

But this series has made all those good moments seem long forgotten. He took criticism for not using a timeout in the third quarter of Game 1 when Miami scored 46 points to completely turn around that contest. He’s taken heat for not being fiery enough, as well, though players said the problems shouldn’t all be on the coach.

“I think it’s a collective effort,” Celtics forward Jaylen Brown said.

Celtics center Al Horford also had Mazzulla’s back, insisting there’s more than enough blame to go around right now.

“At the end of the day, that falls on each player,” Horford said. “We know what we have to do. We knew the magnitude of this game. As a player, I take responsibility because we didn’t have what we needed to have. That’s what that is.”

The phrase in sports parlance is “losing the locker room.” For a coach, that is often extremely damning — and means either players aren’t motivated, aren’t prepared, or just aren’t listening anymore.

Whether that’s happened or not isn’t clear, and really doesn’t matter. In a city that saw the Red Sox make history from coming from 3-0 down in the AL championship series against the New York Yankees in 2004, the Celtics now need the same miracle.

“I have to be better, figure out what this team needs to make sure that they’re connected, they’re physical and they’re together by the time we step on the floor,” Mazzulla said.

Can it happen?

“I’m not sure,” Mazzulla said.

He’s got until Tuesday night to figure it out.