The Bucks' physicality on defence seemed to surprise the Celtics in Game 1 on Sunday.
The Celtics couldn't recover from a first-quarter lead as Milwaukee cruised to a 101-89 victory.
"Milwaukee deserves it," Boston's Jaylen Brown said "They did well and planned well, but we missed many open shots. We have to be ready for the next one."
The Celtics made 18 of 50 3-pointers but only 10 of 31(32%) in the paint.
The Celtics haven't had a worse paint field goal percentage in a playoff game since the 2017 Houston Rockets in Game 6 of the Western Conference semifinals (27.3%).
Brown was 4-of-13 from the field, while Jayson Tatum was 6-of-18. With a combined 32.3 percent field goal percentage, they had a bad playoff game.
The Bucks' physicality took Boston out of sets early, but coach Ime Udoka said it shouldn't have.
Brown said the playoffs aren't the time to be surprised by an opponent, especially one like Milwaukee.