BOSTON -- On a night when the Celtics honored Hall of Famer Bill Walton, the franchise took a massive step toward claiming a record-setting 18th NBA championship thanks to Kristaps Porzingis reprising Walton's sixth-man role.

Porzingis, playing in his first game in more than five weeks because of a calf strain he suffered in the first round, had 20 points, 6 rebounds and 3 blocks in 20 minutes Thursday night, leading the Celtics to a 107-89 demolition of the Dallas Mavericks in Game 1 of the NBA Finals.

It was Porzingis' first playoff game since the first round, and it came against his former team.

"Kristaps had a monster game for us," Jaylen Brown said. "Just his presence shooting the ball and then him being able to attack those switches and mismatches, he made 'em pay every time, and that's what we need going forward in the series."

Porzingis' status had been the dominant storyline over the 10 days between the end of the Eastern Conference finals, which saw the Celtics sweep the Indiana Pacers, and the start of the NBA Finals. He hadn't played since Game 4 of the first round against the Miami Heat.

Porzingis returned to practice last Friday, and while he confirmed at media day Wednesday that he would play, he admitted he wasn't sure what his conditioning level would be until he played in a game. Things became interesting when, half an hour before Thursday's tipoff, Boston announced that Al Horford would remain in the starting lineup, meaning Porzingis would come off the bench.

But then, with just under 10 minutes remaining on the pregame clock and as most of the rest of the Celtics were on the court warming up, Porzingis was shown on the jumbotron walking out to the court. He received a gigantic ovation from the crowd.

If there were any doubts about how Porzingis would look once he got into the game, they quickly disappeared when he checked in with just over seven minutes remaining in the first quarter, emulating Walton, who won the league's Sixth Man of the Year award in helping the 1986 Celtics win the title.

"I think, obviously, what helped me is just even from the walk out before the game and then getting on the court, getting that kind of support was unreal," Porzingis said of the crowd. "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, and it paid off, and we got the job done tonight and had a good game, and happy about the result."

Porzingis put on a clinic in the first quarter. He scored his first points after drawing a foul from former teammate Luka Doncic after posting him up near the elbow, and then hit a jumper on the next possession from basically the same spot. That kick-started the Celtics, who outscored the Mavs 25-9 over the rest of the quarter after Porzingis checked in and led 37-20. Boston would never relinquish the lead.

Porzingis finished the quarter with 11 points, 3 rebounds and a pair of blocks, including a tremendous one in space against former Celtic Kyrie Irving, who had a dismal game and finished with 12 points on 6-for-19 shooting.

The crowd roared its approval at every movement Porzingis made.

"He's a matchup nightmare," Derrick White said. "Even when you play good defense, he doesn't really see you. It's been unreal just watching him all year. Then defensively, he uses his size well, just impacts the games in so many different ways. He changes us, and he makes us a better team."

The return of Porzingis allowed the Celtics to stick to the eight-man rotation they used throughout the season, with the star perimeter quartet of Jayson Tatum, Brown, White and Jrue Holiday; Horford and Porzingis splitting minutes at center, plus some cameos on the court together; and guards Payton Pritchard and Sam Hauser coming off the bench.

The Celtics also stuck to their script and dominated at the 3-point line, shooting 16-for-42 and outscoring the Mavericks by 27 from behind the arc. Dallas shot just 7-for-27 from deep.

That happened, in part, because the Celtics -- unlike most teams -- can stay at home on Doncic and guard him without sending double-teams. Though Doncic had a terrific night, finishing with 30 points and 10 rebounds, he had just one assist, his fewest in a game this season and his fewest ever in a playoff game.

Dallas had only eight assists and committed 11 turnovers.

More than anything, though, Game 1 was about Porzingis and his return against his former team. On the biggest stage of his basketball life, his night went without a hitch, in his game and with his calf.

"Tonight was an affirmation to myself that I'm pretty good, you know?" Porzingis said. "Maybe I'm not perfect, but I'm pretty good and I can play like this and I can definitely add to this team."