BOSTON -- The Milwaukee Bucks punched first.

In Sunday's Game 1 against the Boston Celtics, the Bucks played with their usual level of physicality on defense and it seemed to catch Boston off guard.

After jumping ahead in the first quarter, the Celtics couldn't find their footing the rest of the way as Milwaukee cruised to a 101-89 victory.

"Give credit to Milwaukee," Boston forward Jaylen Brown said. "They did a good job and had a good plan but we missed a lot of open shots. They hit us in the mouth so we gotta be ready for the next one."

While the Celtics were 18-of-50 on their 3-pointers, the team shot just 10-of-31 (32.3%) in the paint. That's the worst paint field goal percentage by any team in a playoff game since the 2017 Houston Rockets in Game 6 of the Western Conference semifinals (27.3%), and the worst by a Celtics team in the playoffs in any game over the last 25 years.

Brown, who came in with a lingering hamstring injury, shot 4-of-13 from the field while teammate Jayson Tatum was 6-of-18. Combined, the 32.3% field goal percentage was their worst in a playoff game as a duo.

Celtics coach Ime Udoka said the Bucks' physicality was something that took Boston out of its sets early, but that it shouldn't have caught the team off guard.

"We have to obviously play with a little bit better poise and you know, not get rushed at times," Udoka said.

Brown said the playoffs are not the time to be surprised by something an opposing team does, especially considering Milwaukee's history.

"They're the defending champs and we gotta come out and play basketball," Brown said. "We have to be ready to go no matter what the injury is or whatever it is that we're dealing with. You gotta have everybody connected and mentally locked in and be ready to leave it all out on the floor."

Milwaukee played effective defense throughout the game, holding Boston to 33.3% shooting overall. It was only the seventh time this season -- postseason included -- that Boston didn't have at least one starter shoot over 50% from the field, and it was only the second time it has happened since the start of the 2022 calendar year.

Boston made just 10 2-point field goals in total, the second-fewest ever by a team in a playoff game. Boston had never made fewer than 14 2-point shots in a regular-season game or a playoff game prior to Sunday.

"We need multiple penetrations, multiple paint touches and they did a good job as far as that," Udoka said. "I think Jaylen and Jayson didn't have their best night offensively, missing ones that they normally are going to make, so those things combined obviously increase those numbers."

While Brown was dealing with his hamstring injury, the Celtics also suffered a scare at the end of the first quarter when Defensive Player of the Year Marcus Smart briefly had to leave the game.

With 1:12 left in the first half, Smart drove to the basket against Bucks guard Pat Connaughton. While Connaughton was called for a foul, Bucks guard Jevon Carter made contact with Smart's right shoulder.

After the whistle blew the play dead, Smart immediately grabbed at his shoulder and let his arm hang down as he called for a Celtics trainer. At halftime, the Celtics said Smart would be able to return to play despite the injury being a stinger, but they also mentioned a right quad contusion.

In the third quarter, Smart took shots to his quad on back-to-back possessions. He was limping at times through portions of the third quarter and had to get on the exercise bike at the end of the third in order to stay warm for the fourth quarter.

Smart missed six games with a right quad contusion earlier in the year, so playing through the pain is something he's done this season. He told reporters on Jan. 23 he was kneed by Domantas Sabonis on Jan. 10 and said his quad swelled up after getting hit in the "perfect spot."

"I think the quad bothered him more than the shoulder," Udoka said. "But you know he's gonna play through a lot and it could affect him. ... He's getting iced down now, so he'll be okay."

ESPN Stats & Information contributed to this story.