Boston’s Jayson Tatum took his share of the blame for the Celtics third-quarter collapse in Game 1 against the Miami Heat on Tuesday. “I’ll be the first one to say I’ll take the blame for that. I’ve got to lead better. I’ve got to play better, especially in those moments,” Tatum said. Lynne Sladky/Associated Press

During a brutal third quarter, the Celtics looked like a hitter who’d been feasting on minor league pitching for weeks only to get a comeuppance against big-league arms.

After looking terrific sweeping the Nets and knocking off the Bucks, the Celtics came out of halftime and watched the Heat turn an eight-point Boston lead into a 17-point deficit at staggering speed in Game 1 of the Eastern Conference Finals. The Celtics looked overmatched against a Miami team that’s had their number before. Boston rallied to make it closer in the fourth, but it was clear shortly after intermission that the Heat were going to win this game.

The Heat went on to a 118-107 win over the Celtics in Game 1, with Game 2 on Thursday in Miami

Boston hopes Marcus Smart, who missed Game 1 with a foot sprain, can return for Game 2 on Thursday. Lynne Sladky/Associated Press

All the reasons to be concerned about the Celtics, specifically in Game 1, proved warranted. Even though it was ugly at times, a loss wasn’t that surprising. Playing without Marcus Smart (injury) and Al Horford (COVID-19 protocols), the Celtics hit a wall in the second half after the short turnaround from beating the Bucks in Game 7 on Sunday.

Like good teams do, they came out aggressively trying to overcome the absences of their veteran starters. But when the well-rested Heat ratcheted up their intensity and physicality in the second half, the Celtics weren’t able to match them. Even with a full roster that wouldn’t have been surprising.

Despite the loss, it wasn’t all bad for Boston. Robert Williams looked terrific in the first half and impacted the game on both ends with 18 points, nine rebounds and two blocks in his 28 minutes. He left the game with a fourth-quarter injury. But if Coach Ime Udoka is right and it was just a cramp, he’s likely to get better over the course of the series as he gets back into rhythm. He’ll either help counteract Horford’s absence or create depth and versatility when he returns.

Aaron Nesmith, who hadn’t played meaningful minutes since March, proved if the Celtics need him, he can give them a temporary injection of energy as well as some athletic defense. If they want to limit Smart’s minutes when he does come back, Nesmith can potentially fill a few of them.

Still the Celtics would benefit from Smart trading the yacht owners outfit he wore on the bench Monday for a Celtics uniform. The Heat won’t be able to pick on him defensively the way they did Payton Pritchard.

Even if neither Smart nor Horford return for Game 2, the Celtics will be much better prepared to play without them with some additional time to prepare.

Jayson Tatum had 29 points and eight rebounds, but was 1 for 7 from the floor with six turnovers in the second half as he looked as flat as anyone after intermission.

“I’ll be the first one to say I’ll take the blame for that. I’ve got to lead better. I’ve got to play better, especially in those moments,” Tatum said. “I think obviously I don’t want to turn the damn ball over and [expletive] like that. But I guess throughout the course of a game, things happen, and they go on runs. That’s what they did.

Throughout the course of the playoffs, we’ve done a great job of responding to runs after calling timeout, things like that. But for whatever reason we didn’t today. … I’m just looking forward to responding next game.”

The Celtics got better in the Brooklyn series from start to finish. After each loss to Milwaukee, Boston changed and improved. This Celtics team has shown an ability to adapt. Udoka expected that wouldn’t change as his team prepared for Thursday’s Game 2.

“Nothing that we can’t clean up,” he said. “We’ve been doing it all season, especially in the second half, and in the playoffs.”

Jaylen Brown found some rhythm in the final 12 minutes (15 of his 24 points came in the fourth quarter) but by then, it was too late to matter.

“We come out and we play basketball,” Brown said. “Most of the time we play good basketball. Tonight they got the best of us. We do a good job of responding and answering runs for the most part. It’s a part of the game. Give credit to Miami; they outplayed us tonight, especially in that third quarter.

“I’ve got to be better. We’ve got to be better, be more poised in that situation. We’ll go look at the film and we’ll come back.”

BOSTON COACH Ime Udoka was expected to speak to the media Wednesday morning for availability, but he was a late scratch because of a non-COVID illness, according to the team.

It’s unclear as of this point on whether the illness will affect Udoka’s status to coach Game 2 of the Eastern Conference finals against the Heat. However, he is still scheduled to speak to the media Thursday, which should be a positive sign as of now.