BOSTON — There was a time when Celtics fans were excited about Kyrie Irving. They can recall the summer of 2017, when Irving forced his way out of Cleveland and landed in Boston, where he delighted in the Celtics’ illustrious past and pledged to do what he could to help the team win.

But over Irving’s two seasons with the Celtics, all that communal excitement morphed into a bunch of different stuff: tolerance as he struggled with injuries, then impatience as he criticized teammates, then something that resembled rage as it became clear that he and Boston were bound for a divorce.

On Sunday afternoon, Irving was back in Boston, where a fervent crowd at TD Garden christened Game 1 of the Celtics’ first-round playoff series with the Nets by booing Irving at every opportunity. They booed him when he emerged from the visitors’ tunnel for warm-ups. They booed him during introductions. They booed him whenever he touched the ball. And he nearly silenced them with another tour de force in a career full of them.

But in the opener of a best-of-seven-game clash between teams with outsize goals, Jayson Tatum sent the arena into a state of pandemonium with a layup at the buzzer that gave the Celtics a 115-114 win. Game 2 is in Boston on Wednesday.

Irving was spectacular. He finished with 39 points and 6 assists, and his 3-pointer with 45.9 seconds left put the Nets ahead by 3. In the process, he reminded Boston why the city wanted him in the first place, while underscoring all the bitterness that has followed.

The Celtics were undaunted in the final minute, though, and after Jaylen Brown drove for a layup, the Nets’ Kevin Durant missed a long 3-pointer. At the other end, the Celtics’ Marcus Smart found Tatum, who spun past Irving for a layup with the clock winding down. It was his easiest bucket of the night.

“I think that’s kind of a microcosm of our season: guys moving the ball, playing unselfish,” Celtics Coach Ime Udoka said. “It all came together on the last possession.”

Tatum finished with 31 points, and Brown had 23. Smart had an astounding all-around game, collecting 20 points, 7 rebounds, 6 assists and 2 steals. Durant had 23 points and shot just 9 of 24.

For the second straight postseason, the Nets and the Celtics are meeting in the first round. The last time, the Nets advanced in five games in a series that only inflamed the dynamic between Irving, who appeared to stomp on the Celtics’ logo at midcourt, and Boston fans, one of whom chucked a water bottle at him.

That series also helped spur significant change in the Celtics’ organization. Brad Stevens moved to the front office after eight seasons as the team’s head coach. His job was filled by Udoka, a longtime N.B.A. assistant and Gregg Popovich disciple who seems to have unlocked the collaborative potential of Tatum and Brown. Remember when the Celtics had a losing record, 23-24, in late January? They closed the regular season by going 28-7.

Udoka entered the series uniquely familiar with the Nets. Last season, as one of Coach Steve Nash’s assistants, Udoka got to know Irving and Durant — and their gifts.

“They’re high-level scorers, and they’re going to get what they get,” Udoka said before Sunday’s game. “Just got to keep them in check and not let them go off.”

Amid a sloppy, foul-marred start, the Celtics’ top-ranked defense gave the Nets fits, forcing seven first-quarter turnovers. The game’s assembled stars — Irving, Durant, Brown and Tatum — combined to miss 12 of their first 14 field-goal attempts.

Irving got going early in the second quarter with a pair of 3-pointers, the second on a pull-up in transition. The game was tied at 61 at halftime before the Celtics began to roll — a jolt that was predictably predicated on their defense. Late in the third quarter, Jaylen Brown blocked the Nets’ Bruce Brown at the rim, then raced away to convert a layup at the other end. Then the Nets took their turn, but Tatum blocked a jump shot by Durant, then hit a 3-pointer to extend Boston’s lead to 11.

Irving was virtually unstoppable in the fourth quarter, scoring 18 points on 7 of 9 shooting, which set the stage for the game’s dramatic conclusion.

The Nets secured the No. 7 seed in the Eastern Conference playoffs by defeating the Cleveland Cavaliers in the play-in tournament on Tuesday. The Celtics had an even longer layoff, with a full week to prepare, since they closed out their regular season on April 10 as the No. 2 seed. Before Sunday’s game, Udoka dismissed a question about rust.

“The vibe with the team is like business,” he said, adding: “We have a nice calm about us where I don’t feel like we’re anxious or going to be nervous out there at all.”

The Celtics were without Robert Williams, their rim-protecting, fourth-year center. Williams was having a breakout season when he tore the meniscus in his left knee last month and had surgery. Udoka said the Celtics were preparing as if Williams would not be available for the series, though Udoka did not rule out the possibility — however remote — of Williams could returning. “He’s progressing nicely,” Udoka said.

Before the game, the Celtics’ game operations crew spiced things up a bit on the arena’s video board by flashing a quote from Bruce Brown about how the Nets could “attack” Al Horford and Daniel Theis in Williams’s absence. (The crowd booed.) Horford was terrific on Sunday, finishing with 20 points and 15 rebounds, and he was animated throughout. Williams, of course, would only enhance the team’s championship hopes.

The Nets are used to waiting, too. They waited for Irving to get vaccinated against the coronavirus, and when he was unwilling to do so, they waited for New York City to lift its vaccine mandates so that he could play in home games. Now, the Nets are waiting — still waiting — for Ben Simmons to take the court for the first time since they acquired him in a midseason trade with the Philadelphia 76ers.

Simmons, who has not played since last postseason, has been dealing with a balky back since arriving in Brooklyn, and no one has any idea what he would look like if he were actually to take the floor against the Celtics. On Saturday, apparently for the benefit of reporters who were monitoring his progress, Simmons dunked at practice.

“Make sure you get this,” he said to those who were filming him with their cellphones.

On Sunday, Simmons wore mirrored sunglasses on the visitors’ bench as Irving and the rest of the Nets went about their business in a hostile environment. For one afternoon, at least, and by the slimmest of margins, the Celtics were the more complete team.