As the Celtics improbably transformed into the most dominant team in the NBA, fourth-quarter scores often became so lopsided that their stars could rest and admire the destruction they had inflicted upon their opponent.
Midway through the fourth quarter of Sunday’s conference semifinal opener against the Bucks, though, the scoreboard wasn’t nearly as kind. With his team trailing by 16 points, coach Ime Udoka pushed for some strong possessions, not because a comeback was imminent, but because perhaps it could generate some momentum for Game 2.
Three minutes later, it was even time to abandon that plan, as Udoka pulled his starters from the 101-89 loss at TD Garden.
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“There’s levels to these things,” Celtics forward Al Horford said, “and they were definitely on a different level tonight.”
The Celtics have not navigated many speed bumps in recent months. And after sweeping the Nets in the opening round and entering this series with homecourt advantage against a Milwaukee team that was without All-Star forward Khris Middleton, confidence continued to rise.
But on Sunday the Bucks provided a stark reminder that they are still the defending NBA champions, that they still have former MVP Giannis Antetokounmpo, and that this will definitely not be easy.
It was Boston’s first double-digit loss in its last 24 games, and it left a mark.
“We’ve got to be the harder-playing team,” Celtics forward Jaylen Brown said. “Tonight, they had a chip on their shoulder, they came in with an edge, they were locked in, and they punched us in the mouth.”
On the final day of the regular season, the Bucks passed up a chance to secure the No. 2 seed when they sat their top players and lost to the Cavaliers. They appeared to be ducking a potential matchup with the seventh-seeded Nets, and when the Celtics had a similar opportunity, they did not run.
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Instead, they stepped into the No. 2 slot, swept Brooklyn in four games, and made a statement in the process.
But maybe Milwaukee was simply saying that it did not care who it played, or where. The Bucks are in the conference semifinals, too, and now the road to the next round will go through their city anyway. The Celtics were certainly not panicking afterward, but the message had been received.
“There’s a reason obviously why they were the champs last year,” Horford said. “That level defensively that they have.”
As dangerous as the Nets can be on offense behind Kyrie Irving and Kevin Durant, their defense tends to be as effective as a napkin after a pipe bursts. So the transition to this tall, strong, intense, and unrelenting Bucks group might have been jarring to the Celtics.
Bucks guards constantly picked up ball-handlers full-court, harassing them and bumping them and taking 6-8 seconds off the shot clock before they could even think about getting into their offense.
Then, Milwaukee’s imposing, sturdy front line used its length to flummox Boston’s attack. The Celtics made just 10 of 34 2-point shots.
The Bucks’ approach did create opportunities to drive and kick, and the Celtics responded by launching 50 3-pointers, tied for the most in a non-overtime game this season. They made 18 of them — not horrible, but also not enough.
“To have 89 points and a lack of penetration and paint points is obviously alarming, so we have to figure that out,” Udoka said. “But we know who they are defensively, and I think their physicality more than anything caught us off guard.”
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Celtics stars Jayson Tatum and Brown both appeared tentative and uncertain on offense. They scored 21 and 12 points, respectively, and combined to make 10 of 31 shots and commit 10 turnovers.
When Middleton was ruled out for the Bucks because of the knee sprain he suffered in Game 2 of his team’s opening-round series against the Bulls, the Celtics’ chances of smothering Antetokounmpo improved.
Although he made 9 of 25 shots Sunday, he impacted the game in other ways, such as gliding through the lane and finding open shooters. Antetokounmpo finished with 24 points, 13 rebounds and 12 assists.
“I think we helped unnecessarily at times when we had some good matchups on him,” Udoka said. “If he’s going to shoot a fadeaway jump shot, we’ll live with that shot. I think we rotated there, and our rotations weren’t sharp.”
Jrue Holiday added 25 points and 10 rebounds for Milwaukee.
The Celtics had an encouraging start. They forced seven turnovers in the first eight minutes, blitzed to a 22-14 lead and resembled the squad that has run roughshod over the rest of the league in recent months. But the Bucks responded with a 13-2 run.
The Bucks took a 56-46 lead to halftime but created an opening in the third. They missed all eight of their 3-pointers, and Antetokounmpo went to the bench after committing his fourth foul with 3:37 left, his team clinging to a six-point advantage.
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But by the end of the quarter the Bucks had added two points to their lead. Celtics guard Payton Pritchard missed four 3-pointers in a row over the first 3:14 of the fourth, and a Boston run never materialized.
“This one,” Horford said, “should hurt a little bit.”
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.