LAKE BUENA VISTA, Fla. -- This season, the Boston Celtics reached the Eastern Conference finals for a third time in four years. But though there was one obvious difference about this trip -- that it came inside the NBA's bubble at the Walt Disney World Resort -- they find themselves facing the exact same result.

After the Miami Heat beat the Celtics 125-113 in Game 6 on Sunday night, the Celtics saw their season -- and their time in the bubble -- come to an end. And as Boston heads into the offseason, it will have a tough time thinking back on a series in which it collapsed in the second half of three of its four losses -- including giving up a 26-6 fourth-quarter run in Game 6 that officially sealed the Celtics' fate.

"They were just more aggressive," Celtics guard Marcus Smart said. "They were getting whatever they wanted ... unfortunately, we didn't combat it. We didn't respond the way we should have.

"It's part of it. We played a really good team. Gotta tip your hat off to those guys. Gotta go back to the drawing board and see what needs to be fixed and come back ready next year."

In looking back on this series, Boston will regret the way it handled those late-game situations. The Heat -- like they did against the Milwaukee Bucks in the previous round -- seemed to get better as games went on. The Celtics, meanwhile, wilted.

That was especially true in Game 6, as Boston took a 96-90 lead with 9:15 remaining, only for Bam Adebayo -- who finished with 32 points, 14 rebounds and 5 assists -- to then turn into a human battering ram, bringing the ball up and attacking the basket repeatedly, to which Boston had no response.

"After we had the lead, Adebayo -- and credit all of them -- but Adebayo decided he's just going to drive the ball put us in a real bind with the shooters around him," Celtics coach Brad Stevens said. "And their physicality is something that I'm not sure we probably talked about enough. But just they're strong, they're physical, they're tough, and him, in particular, dominated that fourth quarter.

"Even the plays where he didn't score, his presence was so impactful. And it put us in a real bind with the ability to guard him. ... They're super physical, super tough, very, very savvy. And I think they're the best team in the East, and deserve to be representing the East the way that they've played."

There's little arguing that, given the Heat are now 12-3 in the playoffs inside the bubble and have now dispatched the league's best team in the regular season, the Bucks, and the Celtics. But that still won't change the fact for Boston that, for the first time in these three trips to the penultimate round of the playoffs, this was a series that Boston was favored to win -- and did not.

And, as Boston heads into the offseason, that is something that will weigh on the minds of the Celtics -- and, in particular, on rising star Jayson Tatum, who spoke about those expectations after the game.

"That it's not easy," Tatum said, when asked what he learned through this experience. "It's tough. If you want those expectations, if you want to be that guy that's capable of doing those things, you've got to go through some tough things, some ups and downs, some stuff I can learn from.

"I think I can learn a lot moving forward, from this season, this series. I'll grow from it."

There was so much about this season that was a success for the Celtics. After the utter dysfunction that plagued them last year, with locker room infighting spilling out into postgame media sessions on a regular basis throughout the season, this year's team had almost none of that.

In fact, arguably the only moment like that came in the bubble, and in this series -- and only after Boston had blown both of the first two games in this series with those second-half collapses, digging a hole the Celtics ultimately were unable to climb out of.

"I really appreciated the way that they played basketball all year," Stevens said. "I really appreciated the way they competed. I really appreciated the way they blocked out stuff that didn't matter. I really appreciated the way that they inspired with their voice while they were here, and before. I appreciated the way they empowered all the different NBA employees that weren't here, including Celtics employees, and everybody else that benefited from them putting everything they had into this, and I appreciated the way they played and found joy and stayed together.

"We had one minor dustup. That's pretty good for a calendar year with a group. It's pretty amazing if you think about it."

Meanwhile, a group that entered the season with seemingly less talent than what the Celtics had assembled the year prior was far more cohesive on the court, as well. Tatum blossomed into an All-Star. Jaylen Brown, who was terrific in Game 6, finishing with 26 points on 10-for-17 shooting in 40 minutes and avoiding what looked like it could have been a serious knee injury in the fourth quarter after an awkward fall on a fast-break dunk.

Kemba Walker was everything Boston hoped he would be in replacing Kyrie Irving, Smart took another step forward, Daniel Theis was a revelation and Gordon Hayward looked back to being near the same player he was before his devastating left foot and ankle injuries two years ago.

Ultimately, though, Boston fell short of where it hoped to go. It took until Games 4 and 5 of this series for the Celtics to finally solve Miami's zone defense. Boston had no answer for Adebayo, who, as Stevens said, overwhelmed them with his physicality. Tatum had an inexplicable scoreless first half of Game 4, while Hayward was a nonfactor in Game 6.

And, both in the Eastern Conference semifinals against the Toronto Raptors, as well as in this series, the Celtics had repeated lapses in concentration that caused them to both start games slowly, and fade out of them down the stretch.

The Celtics were able to -- just barely -- survive those lapses against the Raptors. They were unable to against the Heat.

That was what made the fourth quarter of Game 6 so fitting, as it proved to be the same formula that had doomed Boston time and again over the past few weeks.

It was a fatal flaw the Celtics were unable to overcome.

"Just really getting behind early, starting games slow, starting the series slow," Smart said when asked what he would regret most about how this series unfolded. "But other than that, that's it. I'm proud of my guys. I'm proud of the way we fought. I'm proud of the way we didn't fold, we didn't lay down.

"We had plenty of chances where it could have been over early for us, but we stayed with it, played a great Miami team -- great coaching staff, great players -- and I give credit to those guys. But I'm proud of my guys, and, like I said, just starting the games slow, starting the series slow."

The Celtics now head into the offseason with a group that, for the most part, should return. Hayward has a hefty player option for next season that he will likely pick up. Besides him, all of the core members of Boston's rotation will be under contract for at least next season.

The biggest move Boston will have in front of it will be whether to offer Tatum a max contract extension -- one the team is expected to make. Tatum, though, said that wasn't something he was thinking about right after his season had come to an end.

"That's a tough question to answer," Tatum said. "I haven't even thought about that yet. I was just focused on this season.

"The front office and my agent gotta talk about it. But I'm not thinking about that right now. We just lost a series. Just thinking about the guys in the locker room and the games. That's what I'm thinking about. Stuff like that, going to happen, if it happens, [is] not really my concern. I'm not even thinking about that.

"I'll think about the great season we had, the great players, great job by everyone. It was a hell of a year, and I enjoyed it. I appreciate everybody. This was fun. I'm not really thinking about the other stuff right now."