They blitzed him early with multiple defenders when he received the ball in the post and forced him into turnovers.
They were physical when he drove inside, crowding the paint and obstructing his path to the rim.
The Celtics just couldn't overcome what Milwaukee collectively did to them when they had the ball.
Boston produced its worst offensive performance of this postseason during its 101-89 loss to the Bucks on Sunday, shooting a playoff-low 33% (28 of 84) with just 21 assists and a playoff-high 18 turnovers.
"That's what they try to do, deterring you from driving to the basket," Celtics coach Ime Udoka said. "And for the most part our decision-making wasn't great."
Boston's lowest field goal percentage of the regular season was 34.7% against the Clippers in late December. Including Sunday's loss, the Celtics are now 0-9 this season when shooting less than 39.6%.
Milwaukee did it by limiting Boston's top two scorers, Jayson Tatum and Jaylen Brown, to 21 and 12 points respectively, on a combined 10-of-21 shooting.
"I think they just kind of sped us up," Tatum said. "We just got to do a better job of just doing what we want to do and not letting them dictate that over the course of the game."
Brown said he thinks the looks they got were good for most of the game.
"Basketball. It happens," he said. "We missed a lot of open shots. They hit us in the mouth. Got to be ready for the next one."
Still, Boston had its opportunities.
Antetokounmpo picked up his fourth foul with 3:37 left in third quarter and the Bucks leading 72-64.
But Boston failed to take advantage.
Derrick White's 3-pointer cut Milwaukee's lead to 76-67 with less than a minute left in third. But that was followed by an 11-3 Bucks run that gave them an 87-72 lead early in the fourth in which Celtics went more than four minutes without a field goal.
Antetokounmpo returned in the fourth and was able to pick up things, punctuating his night by throwing a pass to himself off the backboard for a dunk that put his team in front 94-78.
He finished with his second career playoff triple-double, posting 24 points, 13 rebounds and 12 assists.
Giannis Antetokounmpo and the Bucksscore a huge away win in the East semis.
Another reason Boston's rhythm waned was because of a pair of injuries to point guard and top distributor Marcus Smart.
With 11 seconds left the first half, Smart ran straight to the locker room in obvious pain and holding his right shoulder after driving on Pat Cannaughton and banging his shoulder against Jevon Carter, who tried to swipe the ball. The team called it a shoulder stinger and right quad contusion.
Smart was the last Celtics player to return to the court for warmups before the third quarter but was able to start. He played 33 minutes but finished with just 10 points on 3-of-11 shooting with six assists.
"He took a few hits. Took two in the first half. I think the quad bothered him more than the shoulder. But he's gonna play through a lot," Udoka said. "It probably affected him some. … But he'll be OK."
The Celtics responded much better in the second half, yielding only 45 points.
Udoka believes it's a positive sign as they prepare to make adjustments heading into Game Two.
"To lose a 12-point game when we shot so poorly bodes well for us," Udoka said. "In a way it's good to get this dud out of the way offensively."
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