BOSTON — There were definite signs of progress on display throughout the 2016-17 Boston Celtics season.

Isaiah Thomas emerged as a superior scoring threat, Al Horford provided leadership and versatility, and the team won 53 games to earn the No. 1 seed in the Eastern Conference.

But for all of the progress the Celtics made, there was a clear message received when they reached the Eastern Conference finals for the first time in five years.

The Celtics still have a long way to go to compete with LeBron James and his Cleveland Cavaliers.

While James and the Cavaliers are headed to a third straight NBA finals matchup with the Golden State Warriors, the Celtics will start the quest to close a sizable gap.

The Celtics, playing the final three-plus games without an injured Thomas, were taken out by the Cavaliers in five games.

It was no match Thursday night in Game 5 with the Cavs coasting from start to finish and winning 135-102 at the TD Garden.

The 33-point blowout was the most lopsided loss by the Celtics in an elimination game and came after they were beaten by 44 points at home in Game 2 last Friday night.

“We’ve got a long way to go,” said Coach Brad Stevens, whose team has gone from 25 wins to 40, 48 and 53. “We had a great year. We made a run at it. We made progress. But not good enough.

“I think collectively you take away what elite looks like because I think we saw it first-hand, especially in these home games.

“I told our guys we made a lot of great strides, but this pain is part of the path of what we ultimately want to be. This is great learning for all of us, how far we have to go.”

Only a remarkable comeback in Game 3 at Cleveland saved the Celtics from being swept by the Cavaliers. After opening a 16-point lead in Game 4, the Celtics couldn’t sustain it, then faltered in Game 5 at home.

They dropped the three Garden games by a combined 88 points.

“It’s the worst part of the year, being eliminated, falling short,” said Jae Crowder. “It’s a pretty hard thing to swallow so I’m pretty down.”

Said Al Horford: “I’m proud of our group and I felt like we grew as the year went on, and it’s not the way we wanted to finish. The team to get past is Cleveland, and right now we’re not there.”

The Celtics never led and were behind 43-27 after one quarter, then trailed by as many as 39.

The Cavaliers never let the Celtics have an opening, taking them right out in the first few minutes.

James became the NBA’s all-time leader in career playoff points, and finished with 35 points, eight assists and eight rebounds while sitting out the fourth quarter.

Kyrie Irving added 24 points, and Kevin Love had 15 points and 11 rebounds.

Avery Bradley topped the Celtics with 23 points. Gerald Green scored 14.

Even with a healthy Thomas, the Celtics would have struggled against the Cavaliers, who are 12-1 in the postseason.

“They’ve been there before,” said Marcus Smart. “We’re a young team. It’s our first time. We made a lot of mistakes you don’t make at this stage and they capitalized. Everybody has to use this as motivation.”

Any mistake the Celtics made, the Cavaliers jumped all over, getting out on the fast break and scoring at will.

“I thought we played a little too haphazard (Thursday),” said Stevens. “Our offense mistakes led to bad defense and it kind of snowballed on us.”

Even though the final was never in doubt, the crowd at the Garden starting chanting, “Let’s go Celtics” in the closing minutes.

“It’s a good step forward as a season as a whole, but I leave with a little bit of a taste in my mouth of not playing as well here,” said Stevens. “Man, you want to play well here because there’s really no place like TD Garden in Boston. The fans at the end of the game, it’s amazing.”