The Celtics demolished opponents at TD Garden during the regular season and vowed that this postseason would not be like the last one, when they stumbled to a puzzling 5-6 home record.
Things have not escalated to that point yet, but there are now at least some warning signs. In the first round, the Celtics were walloped in Game 2 by the eighth-seeded and severely undermanned Heat. Although the Celtics had little trouble pushing the Heat aside afterward, it raised some eyebrows.
Then in Game 2 of these Eastern Conference semifinals on Thursday, the Cavaliers walloped the Celtics in the second half and rolled to a 118-94 win to even this series, 1-1.
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“Obviously,” forward Jaylen Brown said, “it’s an unacceptable performance.”
By night’s end, a building that is usually known for providing home-court advantages had emptied out. Fans who had sprinkled boos down earlier in the fourth quarter had no reason to wait around.
The Cavaliers did not reach the playoffs by accident. But by the same token, they went 0-3 on the road against the inexperienced Magic in the first round, losing by an average of 22.7 points. Now, their first road playoff win has come against the substantial championship favorites.
It might not end up meaning much, but for now it is humbling.
“The world thinks we’re never supposed to lose,” Celtics forward Jayson Tatum said. “We’re supposed to win every game by 25. And it’s just not going to be like that all the time. So, we don’t expect it to be easy.”
The score was tied at 54 at halftime, but the Celtics unraveled after that and were outscored, 64-40, in the second half.
Donovan Mitchell, who has been the best player in this series, was limited to 6 points in the first half before pouring in 23 in the second, when he connected on 5 of 6 3-pointers. He finished with 29 points, 8 assists, 7 rebounds, and just 1 turnover.
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For much of this season, the 3-point line provided the best way to determine the Celtics’ chances. If they made these shots, they would win. If they missed them, things could get dicey. On Thursday night, the margin beyond the arc was simply too substantial.
Cleveland made 13 of 28 attempts, while Boston hit just 8 of 35.
When the Heat set a franchise playoff record by hitting 23 3-pointers in their Game 2 first-round win, the Celtics were confident that it was just an outlier. They turned out to be correct. This loss to the Cavaliers could be a similar blip, or something bigger.
Although the Cavaliers had success from long range, they certainly did not rely on that shot. Instead, they set the tone early by roaring toward the basket and putting stress on the Celtics’ defense that is looking for ways to compensate for the absence of 7-foot-3-inch rim protector Kristaps Porzingis.
The Cavaliers, who are also without starting center Jarrett Allen, scored 60 points in the paint. They were the more physical and forceful team.
Tatum had 25 points, 7 rebounds, and 6 assists to lead the Celtics. He was noticeably more aggressive and attempted a game-high 11 free throws, but he struggled to finish inside when he was not fouled and has yet to truly display his All-Star form during these playoffs.
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After this loss, he was asked about the perception that it has been a challenge for him to find his preferred shots.
“I wouldn’t necessarily say it’s a challenge,” he said. “I obviously wish I would make more shots, but I’ve been in the league long enough that sometimes you just don’t make them and you’ve just got to still continue to take the right shots. And it’ll even out, but [I’m] not really getting caught up on that. I know how to score the ball.”
Derrick White, whose 3-point shooting had carried Boston over the last three games, hit what the Celtics will hope was just a one-game speed bump, going 1 for 8 from beyond the arc.
The Celtics led by 9 points early in the game and appeared positioned for another comfortable evening. Aside from their Game 2 loss to the Heat, they had trailed for a total of just nine minutes during these playoffs.
But behind forward Evan Mobley (21 points, 10 rebounds), the Cavaliers closed the quarter with a loud 25-7 run. The Celtics pushed back in front, 51-43, before the Celtics once again struggled to close a quarter authoritatively, as Cleveland’s 11-3 burst tied the score at 54 at halftime.
Mitchell started the third quarter by hitting a step-back 3-pointer and quickly ensured that Cleveland would never trail again. Celtics coach Joe Mazzulla said the Cavaliers did a good job of freeing up Mitchell with off-ball actions, and acknowledged that Boston’s defense had some miscommunications while trying to slow him.
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The Celtics remained within striking distance in the final seconds of the third quarter before Mitchell shook Tatum with a crossover dribble and drained a 3-pointer from the left arc just before the buzzer. Afterward, he turned and stared at a stunned, silent crowd before walking to his team’s bench.
The Cavaliers shot 54.5 percent from the field as a team, and several Celtics said they believed their own poor shooting both left them more vulnerable at the defensive end, and also may have affected their focus.
“I don’t care if you’re missing shots,” Brown said. “You’ve got to guard the guy on the other end.”
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.