Once Porzingis finally checked in roughly five minutes into the first quarter Thursday — the first time he had touched the court after a 38-day absence — he got straight to work making it clear the wait had been worth it, keying Boston’s 107-89 runaway victory.

“From the walk out before the game and then getting on the court, getting that kind of [crowd] support was unreal,” Porzingis said. “The adrenaline was pumping through my veins and that definitely helped. Obviously, it wasn’t ideal that I was out for such a long time, but I did everything I could to prepare mentally for this moment coming back. It paid off.”

There were no nerves, no rust and no feel-out period for Porzingis, who arrived in Boston last summer thanks to a trade with the Washington Wizards and had only come off the bench once in his eight-year career before Thursday. Porzingis, playing against a Mavericks team that traded him to Washington in 2022 because he proved to be a poor fit with Luka Doncic, got on the board with free throws by drawing a foul on Dallas’s star guard.

From there, he was off, hitting several midrange jumpers over smaller defenders and throwing down a powerful two-hand dunk against Dereck Lively II to lead a decisive 15-2 run late in the first quarter. On the defensive end, Porzingis drew loud cheers for blocking Kyrie Irving and an even louder ovation for swatting a Josh Green dunk attempt shortly before the end of the first quarter.

Porzingis, invisible for most of Boston’s playoff run, was everywhere as the Celtics took a 37-20 lead into the second quarter and extended their margin to 29 points before entering the break with a 63-42 advantage. Porzingis paced all scorers with 18 first-half points and finished with 20 points on 8-for-13 shooting, six rebounds and three blocks in 21 minutes. His performance was so resounding that Boston was able to rest him for the final eight-plus minutes of the game.

“I just thought [bringing him off the bench] gave us the best chance to win,” Mazzulla said. “He was great on both ends of the floor: defensive execution, fighting for his spot in the offensive end, being physical and making plays. I thought he played great. That was the K.P. who helped us get to where we are today.”

This was a dream opener for the Celtics, one that validated their trust in acquiring Porzingis, who had never previously played in so much as a second-round playoff game, and eased concerns that they might struggle to ramp up after their cakewalk through an injury-ravaged Eastern Conference. Boston was the more poised team from the start, capitalizing on several early Doncic turnovers and making life difficult for Irving, who was booed often by fans who vividly remembered his disappointing two-year Celtics tenure.

The balanced scoring attack Dallas displayed while racing through the Western Conference was reduced to a one-man show by Doncic. Irving scored just 12 points on 6-for-19 shooting, and the Mavericks shot just 3 for 13 on three-pointers before halftime. Though Doncic posted a game-high 30 points and valiantly led a 22-9 third-quarter push that cut Boston’s lead to eight, Dallas never mustered enough support around him.

Jaylen Brown countered Doncic’s flurry with a well-rounded performance of his own, posting 22 points, six rebounds and two assists. With Doncic catching a breather shortly before the fourth quarter, Brown drilled a three-pointer to push Boston’s lead back above 20 points. Making full use of that breathing room, the Eastern Conference finals MVP blew a kiss and let out a long and loud scream toward the sideline crowd during a 14-0 Celtics run.

“When the [Mavericks] cut it to eight, that’s when the game started,” Brown said. “I liked how our team responded. We stayed composed. Offensively, we got to our spacing and were able to push that lead back out. This is a moment where our experience shines through. Just breathe. Just keep playing basketball.”

Dallas will look to correct aspects of its stilted offense before Sunday’s Game 2, especially its clunky ball movement and lack of corner three-pointers. The Mavericks registered just nine assists against 11 turnovers, the first time this postseason they had more giveaways than assists. As a result, Dallas shot 7-27 (25.9 percent) from deep and only four Mavericks scored in double figures. Mavericks Coach Jason Kidd emptied his bench with nearly five minutes remaining.

“The [Celtics] really rely on their great defensive ball pressure and one-on-one defenders,” Irving said. “They funnel us into certain areas. It’s uncharacteristic for us to have only nine assists. The ball has got to move a little bit more. That starts with me just being able to push the pace, get us some easy opportunities and adjust to what they are throwing at us.”

Indeed, Irving endured a night to forget. He launched a jumper that glanced off the rim, dribbled the ball off his foot and got whistled for traveling, among other misadventures. The eight-time all-star’s return to Boston garnered plenty of attention in the run up to the series, but it was Porzingis who claimed revenge against his former team first.

“I will do whatever it takes to help this team win,” Porzingis said. “In this situation, [coming off the bench] made sense. I didn’t care. I knew I could prepare to come off the bench, which is something different for me. I stepped into that role and embraced it. Tonight was affirmation to myself that I’m pretty good [health-wise]. Maybe not perfect, but pretty good.”