By Matthew Geagan, CBS Boston

BOSTON (CBS) — The Celtics are leaving the NBA’s bubble in Orlando much sooner than they wanted, falling to the Miami Heat 4-2 in the Eastern Conference Finals. Boston’s season ended with a 125-113 loss in Game 6 on Sunday night, a disappointing end to an otherwise promising season.

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The Celtics were the better team than Miami entering the series, but the Heat outplayed them in nearly every critical moment. Boston blew leads and couldn’t cool off Heat runs throughout the series, and letting another fourth-quarter advantage slip away in Game 6 leaves a pretty bad taste in terms of how the Celtics season ended for Boston. The C’s had a knack for rising to any challenge during the regular season and their two previous playoff series, but all of that came to a screeching halt against Miami.

Reaching the Eastern Conference Finals was the ceiling most people placed on the Celtics when the season got underway, but the bar kept getting raised throughout the season. Now that the run is over at the hands of the five-seeded Heat, it’s a big disappointment and, in many ways, a gigantic missed opportunity to finally compete for a title. Boston has now lost in the East Finals three times in the last four years.

But the season should not be seen as a failure. A missed chance? Absolutely. Should Brad Stevens and players be criticized heavily for not being able to hold leads in Games 1, 2 and 6? You betcha.

But the season as a whole was not a failure.

When the 2019-20 campaign tipped off for the new-look Celtics, with Kemba Walker in place of the departed Kyrie Irving and Al Horford gone for Philadelphia, the expectations were modest. Even though Jayson Tatum and Jaylen Brown were on the rise, Boston was seen as good but not great. Most put the Celtics in that second-tier of Eastern Conference teams, below the likes of the Milwaukee Bucks, Philadelphia 76ers and Toronto Raptors.

Then the Celtics went out and continually upped their own ante during the regular season. Kemba fit better than anyone expected, and Tatum and Brown both enjoyed new levels of success. Tatum made his first All-Star team and rose to superstar status with some monster performances throughout the year — including the postseason.  Brown was solid all-around and flirted with an All-Stat spot as well, living up to the big extension he received from Boston just before the season.  Gordon Hayward had his best season in green, Daniel Theis became a beast away from the ball, and the Celtics solidified themselves as one of the league’s best defensive squads throughout the year.

They beat the 76ers (a first-round sweep) and the Raptors (a hard-fought seven-game battle) in the playoffs on their way to the Eastern Conference Finals, before losing to Miami mostly on their own doing or undoing. The Celtics had their chances in Game 6 and throughout the series, but the Heat always came up bigger. Whether it was making clutch shots or a big stop on defense, the Heat found a way while the Celtics did not.

Disappointing play led to disappointing — and infuriating — results, and now the Celtics are heading home, coming up short of their goal of bringing a title back from Orlando.

“I thought our guys tried to play physical, but it’s more of a credit to [the Heat], nothing against our guys. They gave it everything they had,” head coach Brad Stevens said after Boston’s Game 6 loss. “Miami was great and for whatever reason, and it was a myriad of reasons, we just couldn’t solve them late.”

“It’s a tough pill to swallow but we fought hard and that’s all we need to ask for,” Walker said after the game. “I’m proud of all these guys. It’s been a really fun year to be around these guys.”

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This was the most playoff basketball that Walker has enjoyed throughout his nine-year NBA career. He had a lot to do with all the “fun” that the Celtics enjoyed throughout the season, though many will remember his first year in Boston for his playoff struggles. Now he’s even more thirsty for next season.

“We’ll get better. We have a lot of talent here,” said Walker. “This was our first year together so we’re going to have time to grow and this is going to be a fun group for a few years.”

Brown was Boston’s best player in Game 6, leading the way with 26 points. He had a tough time putting his emotions into words after Sunday’s loss.

“It’s still kind of raw,” he told reporters. “There are a lot of emotions that I’m trying to process. At the end of the day, they won and we lost. We came up short and it stings.

“We feel like we underachieved, but it’s still no reason to hang our head,” added Brown. “We had a really good group, a young group and talented group that will improve. It stings, but I’m still very, very proud of this group.”

“We are competitors, so anytime we lose a game, especially an elimination game and a series, its tough,” said Marcus Smart, who scored 20 points in Game 6. “It’s still fresh in my mind. Great competitors, so we have to use this to fuel us to come back stronger and better, mentally and physically and emotionally and psychologically.

“We can’t change the past, so we have to get ready for the future,” Smart added.

While the present stings a bit, the future is still bright for the Celtics. Tatum is a legit star, and though he still has plenty of room to grow, he’s the kind of player Boston can build a title contender around. He shook off an 0-for-7 start in Game 6 to finish with 24 points, 11 assists and seven rebounds, and now he knows what life is like as a team’s clear leader.

“It’s tough. But if you want those expectations, to be that guy that is capable of doing those things, you have to go through some tough things,” he said. “Some ups and downs and things you can learn from. I think I can learn a lot moving forward, from this season and this series.”

Now the focus shifts to next season, and with no clear start date on the calendar, it could be a long one as the Celtics try to regroup after another disappointing end. The majority of the roster will be back, and with a little roster tinkering by Danny Ainge (how about a scorer off the bench???) the Celtics should be right back in the mix whenever the 2021 season arrives.

“I believe in this group. We are capable, good enough to get the job done. We just let a couple slip away,” said Tatum. “I give credit to the Heat, they played great. I love playing with this team, it’s a lot of fun. A lot of guys were so much better this year and throughout the season. To see guys grow and get better, it was just fun. It was fun this year.”

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It was certainly a fun season for the Celtics, but once again it ended a bit prematurely. We caught glimpses of the Celtics as title contenders during the regular season and the playoffs, but they couldn’t put it all together when it mattered most. Now they have to let that linger for the next few months, and use it as more gasoline on their fire when next season tips off.