Boston’s Payton Pritchard, top, battles Milwaukee’s Delon Wright for a loose ball during the Celtics’ 119-108 win over the Bucks on Monday in Boston. Michael Dwyer/Associated Press

The Milwaukee Bucks prepared for this.

They knew how dangerous Payton Pritchard could be when he shot from 3-point range, especially in end-of-quarter heave situations. They devoted pregame prep time to making sure Pritchard didn’t beat them, despite the fact he’s a bench player on a Celtics team that features Jayson Tatum, Jaylen Brown and two other top-50 starters.

“It’s so funny – it’s not – but this morning I literally circled his name,” Bucks Coach Doc Rivers lamented. “I was like, ‘This guy comes in and he’s a game-changer.’”

Rivers was right about that.

Pritchard was, in Brown’s words, “a killer” against Milwaukee on Monday night, hitting eight of his 12 3s to power the Celtics to a 119-108 win at TD Garden. His sixth triple beat the third-quarter buzzer and helped turn what had been a tight game – the Bucks led at the half and were within one basket for most of the third – into a comfortable win for the undefeated Celtics.

It was the latest buzzer-beater from a player who buried several during the Celtics’ championship run, including one in Game 5 of the NBA Finals, and it left Rivers looking downright defeated. The former Celtics’ coach had his hands on his knees as Pritchard received an inbounds pass from Tatum and sprinted past Bucks guard Delon Wright, then sagged his head toward the parquet as the shot went in.

Because, again, the Bucks had prepared for this.

“We had to channel him to the middle. We talked about it this morning,” Rivers said. “End of the quarter, he always tries to break off right. We went over that. So it wasn’t a Pritchard reaction. It was more my reaction that this is something we covered and we were there, and we let him go right. Right when he went right, I didn’t know if it was going to go in. I just didn’t think good things were about to happen, and it didn’t.”

Fittingly, Pritchard referred to the shot as “a gut punch,” saying he’ll always attempt those with no regard for his shooting percentage because he knows they can lead to “crazy” momentum swings.

Bucks star Damian Lillard confirmed that containing Boston’s backup point guard was a pregame point of emphasis for Milwaukee, which watched him hit five 3-pointers in a Celtics-Bucks matchup last season.

“We talked about it before the game started, like, we can’t let him come in and have that type of impact on the game,” said Lillard, who’s known Pritchard since the latter was an Oregon middle schooler. “And he came out and had that type of impact on the game. … He’s a hard worker, he’s a believer and he’s going to believe in himself even if he’s missing shots.

“But if you let him come in and get a couple clean looks and get a layup or whatever, he’s going to have that type of impact on the game, especially when he’s done it for a team that’s won a championship. The belief is going to be there even more. We talked about it, and we just didn’t do a good enough job not allowing that to happen.”

Pritchard finished with 28 points on 10-of-14 shooting in 28 minutes, adding five rebounds, three assists and two steals. His production helped the Celtics withstand a lackluster first half from Tatum and Brown (who came alive after halftime to finish with 30 points) and a third consecutive DNP for fellow bench scorer Sam Hauser, who’s been sidelined with lower back pain.

Since going 1 for 9 in Boston’s opener, Pritchard is shooting a sublime 62.1% from beyond the arc (18 of 29). His 19 3s were tied for the league lead entering Tuesday.

“He’s just tough,” Rivers said. “A little tough dude. He’s that little pest that just gets under everybody’s skin, that keeps coming. Gritty, smart. But he’s talented. He shoots the hell out of the ball. He’s got a knack for stripping guys on rebounds. He knows how to play basketball. He’s perfect for that team.”

WHEN HAUSER went down with the back injury that’s now sidelined him for three straight games, Celtics Coach Joe Mazzulla said the plan to replace one of his top reserves would vary each night.

On Monday, it included Boston’s youngest player for the first time.

Jordan Walsh saw the biggest minutes of his NBA career against the Milwaukee Bucks and earned applause for his performance.

“I thought Jordan played great,” Jaylen Brown said. “His minutes were exceptional for our team. He made some big-time offensive rebounds, had some big defensive possessions. Credit to him being ready. Big game, big moment for a young player playing against a potential playoff matchup. I thought he added and contributed to us getting a win tonight.”

Walsh, who spent most of his rookie season in Portland with the Maine Celtics in the G League, missed two open 3-pointers in the first quarter but helped fuel a third-quarter run that broke the game open. The 20-year-old forward grabbed defensive rebounds on back-to-back Bucks possessions, then tipped in a missed Derrick White 3-pointer to put the Celtics up 87-80.

By the time Boston’s regulars exited the game, the Celtics had outscored the Bucks by 15 points with Walsh on the floor. That number dropped to plus-10 after a few late buckets by Milwaukee backups but still was tied for third-best on the team behind Jayson Tatum (plus-18) and Payton Pritchard (plus-12).

“He played well,” Jrue Holiday said. “He came in and played some big minutes against a good team. I think seeing Jordan getting out there, getting some reps, playing great even though he probably wanted to knock down a couple more shots, but you see what type of player he is. And that’s just a testament to him putting in the work, him taking his time and being able to go out there and concentrate. Again off the court, he acts the same way, very under control, and he does what he’s supposed to do.”

After Walsh’s rough showing at the Las Vegas Summer League, Mazzulla urged him to focus on the less flashy, dirty-work elements of his game. The 2023 second-round pick bought in and enjoyed a surprisingly impressive preseason, winning the end-of-the-rotation spot previously occupied by departed free agent Oshae Brissett.

Mazzulla didn’t call on Walsh in the first three regular-season games – the 2023 second-round pick only played in garbage time as Boston leaned more on double-big lineups in Hauser’s absence – but believed he handled his increased role well on Monday.

“I told Jordan in the locker room: I think just finding out a role and making big-time plays like that is huge,” Mazzulla said. “I thought he had a great defensive play on Giannis (Antetokounmpo) in the post, taking away his tendency, and got fouled going up for an offensive rebound. I mean, you have to win games like that, especially against good teams. Sometimes it’s going to be gritty. It’s got to be ugly. So I thought they did a good job of doing that.”