As the Celtics swiftly dispatched a trio of injury-depleted Eastern Conference foes to reach the NBA Finals, it was easy to forget one essential fact:

This Boston team wasn’t at full strength.

On Thursday night, they were. The result: domination of the media-darling Mavericks.

The Celtics rode a terrific return performance from a now-healthy Kristaps Porzingis and an excellent all-around showing by Jaylen Brown to a 107-89 victory over Dallas at TD Garden.

The emphatic win gave Boston a 1-0 lead in the best-of-seven Finals, with Game 2 set for Sunday night on Causeway Street.

Porzingis scored 20 points on 8-of-11 shooting and added six rebounds and three blocks after missing the previous 10 games — two full playoff series — with a calf strain.

Brown led all Celtics scorers with 22 points (7-for-12) to go along with six rebounds, two assists, three steals and three blocks. The Eastern Conference finals MVP led Boston’s multi-player effort to defend Mavericks superstar Luka Doncic, who was characteristically productive (30 points, 10 assists, 10 rebounds) but didn’t receive nearly enough help from his teammates.

Celtics villain Kyrie Irving, who was loudly and persistently booed in his latest return to Boston, struggled, finishing with 12 points, two assists and no free-throw attempts while going 0-for-5 from 3-point range.

Jayson Tatum had a quiet offensive outing for Boston (16 points on 6-of-16 shooting with five turnovers) but was a team-best plus-21 in his 42 minutes.

Though head coach Joe Mazzulla said Porzingis had no minutes restriction in his first action since April 29, he chose to keep veteran Al Horford in the starting five. The 38-year-old made an immediate impact in his 11th consecutive start, opening the scoring with a baseline dunk.

Horford then proceeded to hit a three, grab an offensive rebound and block an Irving shot before heading the bench at the 7:17 mark of the first quarter with the Celtics ahead 12-11. Porzingis’ entrance drew roars from the juiced-up Garden crowd, which reacted similarly when the 7-foot-2 big man emerged from the tunnel for pregame warmups.

Porzingis’ first points since Round 1 of the playoffs came after he posted up Doncic — with whom he never meshed during their ill-fated time together in Dallas — and drew a foul on a midrange jumper.

That was just the start.

Porzingis scored eight straight points for the Celtics, interrupted only by a Doncic three, as Boston built a 10-point lead late in the first quarter. During that same stretch, he blocked a Jaden Hardy layup and forced misses at the rim by Derrick Jones Jr. and P.J. Washington.

Then, after Tatum buried a 3-pointer for his first points of the night and Irving badly misfired on a triple at the other end, Porzingis took a feed from Payton Pritchard and knocked down a 29-footer. Moments later, he swatted away an Irving jumper, sparking one of many “Kyrie sucks” chants that echoed through the Garden on Thursday night. Sam Hauser followed that up with a quick three in transition.

The Mavs appeared to finally gain some momentum when Irving stripped Tatum to spark a fast break, but Porzingis swooped in to deny a soaring dunk attempt by Josh Green.

Boston held a one-point lead when Porzingis checked in for the first time. By the time Horford relieved him at the end of the first quarter, that lead had swelled to 17.

It continued to balloon in the second. Porzingis picked apart Dallas’ defense from mid- and long range, including a deep three over Doncic’s outstretched palm. Hauser, Brown added triples of their own, and Brown (twice), Tatum and Derrick White all threw down dunks.

The Celtics led by as many as 29 points before a late Doncic flurry capped by a three over Porzingis. Even after that brief lapse, Boston took a commanding 63-42 advantage into halftime.

Porzingis had 18 points on nine shots at the break, the most by any player in the first half of his NBA Finals debut since 1977.

But the Mavs’ surge to close out the second quarter continued in the third. Led by the indefatigable Doncic, the West winners kept hacking away at Boston’s lead, pulling to within single digits after a 22-9 run to open the second half.

That prompted a Mazzulla timeout, and the stoppage seemed to settle the Celtics. Boston proceeded to score the next 14 points to break the game back open.

Brown spurred that response by drawing two fouls on drives to the basket, draining a corner three and playing excellent defense. He blocked Washington at the rim twice on the same Mavericks possession and did the same to Irving a minute later. Brown accounted for 10 of the Celtics’ 19 foul shots in the game.

Add in threes by Tatum and Horford and a driving Porzingis dunk off a Tatum offensive rebound, and Boston’s lead was back to 20 heading into the fourth quarter.

Dallas never threatened in the final frame, allowing Mazzulla to empty his bench as the satisfied Garden faithful began heading toward the exits.

Doncic was the only Mavericks player to score more than 14 points, and Dallas finished with just nine assists to the Celtics’ 23.