BOSTON — Prior to Game 5 of a tied Eastern Conference semifinals against the Philadelphia 76ers, Celtics head coach Joe Mazzulla said, "It's not much different than the regular season," and his team played like it.

As was the case in Sunday's Game 4 loss, Boston lacked the requisite energy from the jump, and the deficit snowballed to double digits before halftime. Only, this time the defending conference champions never met their rival's competitiveness or made it interesting, falling 115-103 to the brink of elimination.

"That was the first game of the playoffs that we didn't play well, in my opinion," said Mazzulla, leaving out Game 1 and most of Game 4 of this series and Game 5 of their first-round victory over the Atlanta Hawks.

The Sixers take a 3-2 lead back to Philadelphia for Game 6 on Thursday at 7:30 p.m. ET.

Rarely has TD Garden been so lifeless in the playoffs. Boos began in the second quarter, and by the end of the third, when the Celtics watched Tyrese Maxey's uncontested layup, the crowd's contempt reached a fever pitch. In the rare moments they played inspired enough to capture their fans' attention — forcing a 24-second violation down 14 late in the third and cutting the deficit to 11 on Jayson Tatum's and-one early in the fourth — they respectively turned the ball over into a layup and allowed another wide-open 3-pointer.

"The energy wasn't right," said Celtics guard Marcus Smart. "It could've been better, way better. We know that. We understand that. Tonight speaks volumes. It shows that, if you're not ready ... this can happen."

"We've gotten booed before," added Tatum, who scored 36 points, "so it's nothing new. We've been in that position before, and we didn't play well today. The fans could see it. You guys saw it. And we know that."

It was a listless loss in a growing number of them for a team that has led the NBA in net rating over the past two seasons, and there is no shortage of blame to go around. The list will surely start on local sports radio with Mazzulla, the 34-year-old thrust into the top job once Ime Udoka was suspended before training camp for an allegedly improper relationship with a subordinate and lead assistant Will Hardy left for the Utah Jazz.

There are legitimate criticisms, to be sure. The Celtics have stuck with a drop pick-and-roll coverage that has not only allowed James Harden breathing room to fire from distance but given Joel Embiid freedom to feast on his bread-and-butter jumpers. Worst of both worlds. After not calling a timeout on a broken final play that cost them Game 4, Mazzulla said, "I'll definitely learn from that." He expressed a similar sentiment after losing to the Embiid-less Sixers in Game 1. Not what you want to hear from the coach of a contender.

Then, there's the matter of a team that made the Finals with defense as its calling card now branding itself an offense-first outfit. Their 117.3 points per 100 possessions did lead the East during the regular season.

"Our strength is our offensive management," Mazzulla told reporters during a practice session between Games 1 and 2. "This team has been built on defense for a very, very long time. They have the DNA of that, and they're always going to play hard, but we manage the game best with our offensive decision-making."

Might want to rethink that after Games 4 and 5. When the Celtics' 3-pointers aren't falling like they weren't on Tuesday (27.3% on 33 attempts before garbage time) and when the stagnation of matchup-hunting supersedes ball movement, defense could keep them within striking distance, but it is nowhere to be found. They allowed 121 points per 100 possessions in only 11 games during the regular season and won seven of them. They have now given up that many in six of their 11 playoff games, including all five of their losses.

"We gave everything up that they wanted us to give up," said Boston's Jaylen Brown.

This is a fundamental problem, but it is not Boston's only one. The players are not without fault, either. Late-game mental breakdowns like in Game 4 of this series and lulls like in Game 5, when they played as if their talent advantage alone would deliver a victory, have been commonplace for three years running under coaches Brad Stevens, Udoka and Mazzulla. Stevens' in-game adjustments narrowed the margins of error, as did Udoka's blunt force, but ultimately it falls on the players to learn from their considerable experience.

Tatum and Brown are just entering their primes, but they have played 84 playoff games together. If the Celtics are not yet willing to concede that the partnership has a sub-championship ceiling — and it is reasonable to think they can still ascend — Stevens, now Boston's head of basketball operations, will have questions to answer beyond Brown's next contract if his team loses to a team it has previously owned.

Should Smart have the third-highest usage rate in a pivotal game for a team that also boasts Derrick White and Malcolm Brogdon in the backcourt? Al Horford turns 37 years old next month, and Robert Williams III has not played with the same bounce since last year's knee surgeries. And where has Grant Williams gone?

As the final minutes dwindled on their fourth Game 5 loss in five attempts, the Celtics' starters watched from the bench. Horford put his arms around Tatum and Brown and told them something he preferred to keep in the locker room, but surely they can all see what is in front of them: The promise of potentially playing an eight seed in the conference finals for the right to face one of the West's flawed teams for a title.

"The past is the past," Brown said. "We could talk about a lot of stuff that's happened in this series that could've went the other way and didn't. We've got in front of us a great opportunity, and to mull on the past would take that opportunity away from us. We've just got to be ready to play basketball. That's it."

Like Mazzulla, Boston's young stars must learn from their past lapses, and that is not so easy in two days' time. It takes years, and they are six seasons into their championship pursuit. If they cannot correct them for a win-or-go-home Game 6 in Philadelphia, the Celtics at least have to begin wondering if they ever will.