Boston Celtics Coach Joe Mazzulla said after Sunday night’s game: “I just didn’t execute the proper game plan. I didn’t put them in the right mentality to be ready, and it’s my job to make sure that they’re connected and that they’re ready to play, and I didn’t do that.” AP Photo/Lynne Sladky

MIAMI — The Miami Heat fans leaving Kaseya Center were still loud, cheering and clearly audible when Joe Mazzulla took the podium, which was located just steps away from the court, a curtain separating him from the carnage that just took place.

The Boston Celtics coach was still trying to process what just happened, a humiliating and inexcusable 128-102 loss to the Heat in Game 3 of the Eastern Conference Finals that suddenly left his team one loss from elimination. It was the darkest night of the season, one once filled with so much promise but now in real jeopardy of crashing to a devastating end. It seems inevitable now, really.

But with the lights on him, Mazzulla had nowhere to go, nowhere to hide. The first-year coach put it all on himself.

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

Mazzulla was thrust into a very difficult spot in September when he suddenly took charge of a championship-ready team for the suspended Ime Udoka. He handled circumstances admirably through the regular season. But his inexperience is being exposed this postseason, and the spotlight is burning on him now after Sunday’s horrific loss.

In the biggest game of the season – one the Celtics absolutely had to win – their doors were blown off. They were in it for the first six minutes, but the 42 minutes after was their worst nightmare coming to reality.

Mazzulla took the blame – over and over.

“I just didn’t have them ready to play,” he repeated. “I should have – whatever it was, whether it was the starting lineup or it was an adjustment, I have to get them in a better place ready to play, and that’s on me.”

“I have to be better,” he said. “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.”

The Celtics showed little fight after the Heat ran away with the game in the third quarter. How disappointing was that?

“It’s on me to be better for them so that they play harder,” Mazzulla said.

Celtics embarrassed in spineless Game 3 loss to Heat, now face 3-0 series deficit

“I just didn’t have them ready to play,” he continued. “I just didn’t execute the proper game plan. I didn’t put them in the right mentality to be ready, and it’s my job to make sure that they’re connected and that they’re ready to play, and I didn’t do that.”

What was Jaylen Brown’s reaction to hearing Mazzulla didn’t have the team ready?

“I think it’s a collective effort,” the Celtics star said.

Al Horford didn’t let his coach take all the blame. After the Celtics fell behind by 18 in the second quarter, Horford was at the center of the huddle, delivering an impassioned speech. But it clearly had no effect.

The 36-year-old veteran shared in the blame.

“Coach is saying that, he’s being generous,” Horford said. “At the end of the day, that falls on each player. 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.”

Celtics players have stood by Mazzulla – whose interim head coach tag was removed in February along with a contract extension – during a difficult postseason run for the coach in which he’s made several mistakes, some in critical situations. They’ve continued to say publicly that they believe in him, with Marcus Smart at the forefront.

But Mazzulla acknowledged Sunday that there is a disconnect between him and the players. How else to explain what transpired on Sunday night?

“That’s where 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.

What caused the disconnect?

“I’m not sure,” Mazzulla said.

The ingredients for Sunday’s disaster were plenty. The Celtics weren’t ready, they weren’t focused, they didn’t have the mentality to win. And a lot of that came on the defensive end. The Celtics were lost in transition and broken in the half-court, where the Heat had no problem doing whatever they pleased, whether it was open looks from deep or alley-oops for Bam Adebayo.

Jimmy Butler wasn’t even really a factor. He scored 16 points. It was the role players – Gabe Vincent, Caleb Martin and Duncan Robinson, all undrafted players – who took over.

The Celtics were the league’s top defense a year ago and that was the reason why they turned their season around under Udoka and came two wins short of a championship. They haven’t been nearly as good defensively this season even while ranking among the top units during the regular season. They’ve shown flashes, and they looked like they turned a corner defensively in the 76ers series after holding them below 90 points in three games.

But the C’s have clearly lost a step on that side in this series. They haven’t been focused or detailed enough.

“I think some of that defensive identity has been lost, and we have to get that back, and that’s where part of that is on me to make sure we get that back,” Mazzulla said.

“Just these last couple games. I think just the execution, just we’re not connected. Usually at our best, we’re connected, we’re together, we’re physical on the defensive end, and we don’t have that right now.”

Horford agreed the defense has been lost. As the Celtics prepare for a do-or-die Game 4 on Tuesday night, that’s the focus.

“The most important thing is how can we get our defensive identity, how can we get just a clear understanding of what it looks like on the offensive end and just keep it as simple as we can so that we’re connected and we’re physical, that we play with the right mindset and we play together,” Mazzulla said.