The Milwaukee Bucks prepared for this.

They knew how dangerous Payton Pritchard could be when he got 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 head 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 threes 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 half and were within one basket for most of the third — into a comfortable win for the undefeated C’s.

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 ex-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 told reporters. “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,” Lillard, who’s known Pritchard since the latter was an Oregon middle schooler, told reporters. “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 season opener, Pritchard is shooting a sublime 62.1% from beyond the arc (18 of 29) and has to rank among the very early favorites for the NBA’s Sixth Man of the Year. His 19 made threes 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.”