In the final seconds of the first quarter Tuesday, Baylor Scheierman scooped up an offensive rebound with his back to the basket, swiveled and fired up a one-legged fadeaway 3-pointer as time expired.

The ball did not come close to going in, but the moment was notable because that was the only shot Scheierman missed. The Celtics rookie poured in a career-high 20 points — 17 in the second half — to lead the shorthanded Celtics to a 104-96 win over the Nets. It was Boston’s 50th win of the season.

“Obviously it’s a lot of fun when [the game] slows down and it feels like you’re just out there playing free, playing loose,” Scheierman said. “Coach [Joe Mazzulla] has got a lot of confidence in you, players have got a lot of confidence in you. That’s what the game is all about, and that’s how I enjoy playing.”

Entering Tuesday, Scheierman was just 5 for 25 from the 3-point line all season. He topped that mark by hitting 6 of 7.

The first-round draft pick out of Creighton has seemed to lack confidence at times this season. He has shuttled back and forth between Boston and the team’s G League affiliate in Maine, and hit just 1 of his first 15 NBA 3-pointers.

But Mazzulla pushed back against the notion that Scheierman was uneasy. He said it has been clear that he is sure of himself; he just happened to join the defending NBA champions who haven’t needed him much just yet.

“Confidence is kind of an overused term,” said Mazzulla, who appreciated Scheierman’s offensive rebounds and hustle plays even more than his elite shot-making vs. the Nets. “I think in reality, it’s like, this kid just got here. He’s been here for three months. Like, what do you expect him to look like 10 games in playing sporadically? He’s always had that confidence. I think a lot of it is a matter of timing. A lot of it’s a matter of opportunity. And a lot of it is the moments that you get, you’ve got to deliver and you’ve got to impact winning.”

Celtics All-Stars Jayson Tatum and Jaylen Brown sat out against the Nets to rest, and several others who ended up playing probably would have benefited from a breather, too.

But the 24-year-old Scheierman, who had played just 143 minutes all season, was fresh and ready, and it turned out he was exactly what the Celtics needed on a night when the rest of the team went 10 for 41 from the 3-point line.

As Derrick White put it during his walk-off interview that was broadcast on the TD Garden video board: “Honestly, Baylor changed the whole game for us. Shout-out to the rook.”

At the end of the third quarter the Celtics had a well-executed 2-for-1 opportunity, with Scheierman leading the way. He sliced to the hoop for a layup with 29 seconds left, ensuring Boston would get another chance.

After the Nets used their foul-to-give with 1.9 seconds left, Scheierman inbounded the ball to Kristaps Porzingis (25 points, 13 rebounds, 3 blocked shots) from the left sideline.

Porzingis quickly sent it back to Scheierman, whose 3-pointer at the buzzer swished through the net and gave the Celtics a 71-70 lead. It looked like a beautifully designed play, but it was improvised, with White serving as the initial intended target.

“I was surprised [Porzingis] threw it back to me,” Scheierman said, “so I tried to get my feet and balance as quick as possible and just let it fly.”

Scheierman added another 3-pointer at the 10:15 mark of the fourth before the Celtics went more than three minutes without a point and lost its lead.

But Scheierman then hit a catch-and-shoot 3-pointer from the left corner with 7:08 left, and on the Celtics’ next possession he dribbled upcourt and stepped into a deep 3 from the left arc. It bounced off the rim, and there were brief groans of disappointment from the fans. Then the ball took two more bounces and slid through the net, giving Boston an 84-80 advantage.

“One thing you can say about him is he doesn’t play hard — he plays really hard, and I think the fans see that and they appreciate that,” Porzingis said. “And then on top of that, a game like this offensively, just beautiful to watch. And that was a beautiful, beautiful game from him.”

Finally, with 5:30 left, Scheierman shook his defender with a pump fake and stepped into another 3-pointer — his fourth of the quarter — for an 89-82 Celtics lead. Scheierman received a standing ovation when he exited more than a minute later.

During his on-court postgame TV interview, Scheierman was showered with water bottles by several teammates.

The celebration continued in the locker room. Torrey Craig poured some water down Scheierman’s neck, and it appeared he had escaped the larger soaking when he began going around the room and slapping five with his teammates.

But they were just slow-playing his big night. Within moments, the room turned into a water-bottle rainstorm.

“This kind of game, I think, will stay in his memory for his whole career,” Porzingis said.

Adam Himmelsbach can be reached at [email protected]. Follow him @adamhimmelsbach.