BOSTON >> The Celtics made an NBA Finals run nobody saw coming, rising from a team that was three games under .500 in early January under a first-time head coach to being within two victories of winning the franchise’s 18th championship.

After having those hopes dashed in the haze of three straight losses to the champion Golden State Warriors, Boston’s young core is vowing to use the pain as motivation heading into the offseason. While they didn’t win, the Celtics have established themselves as one of the top teams in the Eastern Conference.

“The biggest message was learn from this, grow from it, take this experience and see there is another level to get to,” coach Ime Udoka said after Thursday night’s loss. “Just don’t come back the same as players, coaching staff. Let this fuel you throughout the offseason into next year.”

When Udoka took over the coaching reins from Brad Stevens this past offseason, the Celtics were a team in transition.

Stevens, it seemed, had gotten the Celtics as far as he could during eight seasons on the bench. He took Boston to three conference finals but failed to help its youthful core led by budding All-Stars Jayson Tatum and Jaylen Brown get over the hump.

Enter Udoka, who did just that in Year 1. He challenged the team and harnessed the experience of veteran pieces like Marcus Smart and Al Horford to slowly mold a culture built on defense and unselfishness on offense.

It paid off in the second half of the season as the Celtics turned an 18-23 record into a 51-31 mark at season’s end, good enough to claim the East’s second seed. After a sweep of Brooklyn, followed by back-to-back seven-game wins over defending champion Milwaukee and top seed Miami, Boston seemed unstoppable.

The Celtics took an early 2-1 finals lead on Golden State but ultimately couldn’t sidestep its propensity for self-inflicted mistakes against the Warriors.

The Celtics were 1-7 in the playoffs when committing 16 or more turnovers. They hit the figure in three of their four losses in the finals.

Tatum was woefully inconsistent, averaging 22 points but shooting just 44 of 120 (37%) from the field. He was 5 of 13 from the field with 12 points in the deciding game Game 6 in Boston.

Worse, Tatum became the first player in NBA history with 100 turnovers in a single postseason.

However bitter, he said he’s embracing the lessons he learned.

“It’s hard. It’s hard getting to this point,” Tatum said. “It’s even harder getting over it, the hump, and win it. It’s been a long journey, a long process. That’s what I took from it: it’s tough. You got to take it up another level to do what we want to do.”

The good news for Boston is that their coach has experienced both extremes of the finals. He was a first-year assistant under Gregg Popovich when San Antonio lost to Miami in the finals in 2013, and there the following year when the Spurs beat the Heat to claim the title.

“We improved in a lot of areas but fell short of our ultimate goal,” Udoka said. “Some guys didn’t play their best. That’s going to motivate guys throughout the season. The message is everybody come back better. Let’s not be satisfied. It’s not guaranteed you’re going to be here.”

NO SURGERY

Tatum grimaced and clutched his right shoulder at times throughout the finals, a product of the stinger injury he suffered in the conference finals against Miami.

He played through the pain in the finals, but also insisted it wasn’t a factor in his performance.

As of now, he doesn’t have plans for surgery.

CHANGING OF THE GUARD?

One of the biggest questions this summer will be what Stevens, who is concluding his first year as president of basketball operations, plans to do at point guard.

The trade of Kemba Walker last offseason and jettisoning of Dennis Schroder this season put the responsibility in Smart’s hands. He excelled at times, but his defense-first mindset hurt a team that at times was crying out for a true floor general to open up the floor for Boston’s shooters.

Replacing him would be a tough call as he is also Boston’s emotional floor leader.

Smart, who is under contract through 2025-26 season, is focused only on trying to help the team that drafted him in 2014 to improve.

“We’re young. … Things we went through to get here showed us what we have to come for us in the future,” he said. “I think that’s why we’re confident about the future. We all know what the goal is in the future.”