The Milwaukee Bucks have prepared for this.

They knew how dangerous Payton Pritchard could be when coming from 3-point range, especially in lob situations at the end of the quarter. They dedicated time to game preparation to make sure Pritchard didn’t beat them, despite being a bench player on a Celtics team that includes 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,” complained Bucks head coach Doc Rivers. “I thought, ‘This guy is going to come in and change the game.'”

Rivers was right about that.

Pritchard was, in Brown’s words, “a killer” Monday night against Milwaukee, hitting eight of his 12 three-pointers and giving the Celtics a 119-108 win at TD Garden. His sixth three-pointer beat the buzzer of the third quarter and helped turn a close game – the Bucks led at halftime and were within a basket for most of the third quarter – into a comfortable win for the undefeated C’s.

It was the latest buzzer from a player who buried several during the Celtics’ championship run, including one in Game 5 of the NBA Finals, and left Rivers looking utterly 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 lowered his head toward the floor as the shot went into the goal.

Because here too, the Bucks had prepared for this.

“We had to direct him to the middle. We talked about it this morning,” Rivers told reporters. “He always tries to break away at the end of the quarter. We went through that. So it wasn’t a Pritchard reaction. It was more my reaction that we covered that and we were there and let him go right. When it really started, I didn’t know if it would go in. I just didn’t think good things would happen, and they didn’t.”

Appropriately, Pritchard called the shot a “gut punch” and said he will always attempt these, regardless of his shooting percentage, because he knows they can lead to “crazy” swing swings.

Bucks star Damian Lillard confirmed that Milwaukee placed a pregame emphasis on containing Boston’s backup point guard, as he made five 3-pointers in a game between the Celtics and Bucks last season.

“We talked about it before the game started. We can’t let him come in and have that kind of impact on the game,” Lillard, who has known Pritchard since he was a middle schooler in Oregon, told reporters. “And he came out and had such an impact on the game. … He’s a hard worker, he’s a believer and he’ll believe in himself even when he doesn’t have the opportunity.

“But if you let him come in and get some clear looks and make a layup or whatever, he’s going to have such an impact on the game, especially if he did it for a team that won a championship. Faith will be there even stronger. We talked about it, but we just didn’t do a good enough job not to let that happen.”

Pritchard finished the game with 28 points on 10 of 14 shooting in 28 minutes and added five rebounds, three assists and two steals. His performance helped the Celtics withstand a lackluster first half from Tatum and Brown (who came alive after halftime, scoring 30 points) and a third straight DNP for backup Sam Hauser, who was sidelined with lower back soreness.

Since going 1-for-9 in the season opener in Boston, Pritchard has shot a blistering 62.1% from distance (18 of 29) and has to be one of the very early favorites to be named the NBA’s Sixth Man of the Year. His 19 made three-pointers secured him the league lead on Tuesday.

“He’s just tough,” Rivers said. “A bit of a tough guy. He’s this little pest that just gets under everyone’s skin and keeps coming back. Dark, clever. But he is talented. He shoots the ball like crazy. He has a knack for taking guys down on rebounds. He knows how to play basketball. He’s perfect for this team.”