Until about an hour before tipoff Monday night, it was unclear whether Jayson Tatum would suit up against the Miami Heat.

The Celtics superstar was added to the injury report a day earlier, listed as questionable with a knee issue, and with the All-Star break looming, Boston could have opted to give its most important player an extra night of rest.

Unfortunately for Miami, that’s not the path the Celtics chose.

Tatum was cleared to play, then proceeded to torch the Heat to the tune of 33 points and eight rebounds, with nearly two-thirds of his scoring output coming during a dominant third quarter. Boston shook off an early deficit and cruised to a 103-85 win at the Kaseya Center.

The Celtics were missing two starters in Jrue Holiday (shoulder) and Jaylen Brown (knee), but replacements Sam Hauser and Al Horford went a combined 9-for-16 on 3-point attempts. Hauser was a defibrillator for Boston, awakening its offense after a scattershot first quarter, and Horford added 10 rebounds for his second double-double in his last four games.

All five Boston starters scored in double figures, and the Celtics’ defense overwhelmed a Heat team that was playing without All-Star Tyler Herro and Jaime Jaquez Jr. (both out due to illness) and integrating three new players following last week’s Jimmy Butler trade.

Miami’s 85 points were tied for the second-fewest by a Celtics opponent this season. Boston head coach Joe Mazzulla, whose team improved to 38-16 and an NBA-best 22-6 away from TD Garden, stressed how important Tatum was to that effort.

“He’s not just being defined by scoring,” Mazzulla told reporters in his postgame news conference. “He’s defending at a high level, which is really important for us. He has to rebound. He has to play defense for us to be great, and he’s doing that. When he’s playing connected basketball on the offensive end, he’s not forcing it, he’s finding the right shot, distributing. His potential assists are up, and he’s just making plays. We just need him to continue to play on both ends of the floor.”

The first quarter was an ugly one for the Celtics, who fell behind by double digits while shooting 24.0% from the field (6-for-25) and 11.1% (2-for-18) from 3-point range. Kristaps Porzingis (17 points, nine rebounds, two blocks) spearheaded Boston’s offense with a series of savvy post moves and bank shots, but he and his teammates were ice-cold from the perimeter. Porzingis and Tatum were especially erratic early, missing their first eight combined 3-point attempts.

The Celtics’ defense shifted the momentum late in the first, with help from an unexpected source. Rookie Baylor Scheierman, who’s seen just a handful of non-garbage-time minutes in his young NBA career, checked in at the 2:37 mark and helped force a pair of Heat turnovers with stingy on-ball D.

“I thought we picked up our activity as far as forcing turnovers and deflections in the middle of that first quarter throughout the rest of the game,” Mazzulla told reporters. “Our second unit did a good job of that.”

Boston held Miami scoreless for nearly seven straight minutes, during which their 3-point shooting finally stabilized.

Or, more accurately, Hauser stabilized it. After hitting one triple two minutes into the game, he drilled four more early in the second quarter — on four consecutive attempts — tying his single-game season high with more than 30 minutes to play.

Hauser (15 points, 5-for-8 from three) lacked his usual long-range effectiveness during the first half of the season as he battled a lingering back injury, but he’s looked more like his sharp-shooting self of late. He’s shot better than 50% on threes in nine of his last 10 games, inching his season-long percentage back above 40% — a mark he surpassed in each of his first three NBA seasons.

And when Hauser’s shot is dialed in, the Celtics are tough to beat. They’re 15-2 this season when he makes at least three 3-pointers and 9-0 when he hits four-plus. Over the last two seasons, they’re 42-8 and 24-3, respectively.

“I feel like I’ve been playing well for the last couple weeks, for sure,” the 27-year-old told reporters postgame. “So I feel pretty good.”

Sparked by Hauser, the Celtics erased an 11-point deficit and led by as many as 16 before halftime, outscoring the Heat 39-12 during that game-changing stretch.

On the court for all of it: another core reserve, Payton Pritchard, who played the final seven minutes of the first quarter and the entire second. Though he went just 1-for-8 from beyond the arc in the game, the Sixth Man of the Year front-runner entered halftime on triple-double watch (five points, six assists, six rebounds, two steals), and the Celtics won his 19 first-half minutes by 15 points. He finished with five points, 10 rebounds and eight assists.

Tatum also found a rhythm shortly before half, scoring on three straight Celtics possessions after going without a field goal for the first 21 minutes of game time. Derrick White and Horford each added a 3-pointer as Boston stretched its lead.

The Heat closed the half with a 7-0 rally, capped by Bam Adebayo’s buzzer-beating bank-shot three over Porzingis, and trailed 52-43 at the break. But Tatum blew them away in the third quarter, pouring in 20 points on 8-of-11 shooting.

It was the highest-scoring quarter of the season for the six-time All-Star, and it came two days after he dropped 19 points in the same frame in Boston’s blowout win over the New York Knicks at Madison Square Garden. In this one, Tatum hit three threes, sank an elbow jumper and went 4-for-4 on shots at the rim, tearing apart a new-look Miami lineup featuring Butler trade pickups Andrew Wiggins, Kyle Anderson and Davion Mitchell. He also grabbed four rebounds and assisted on a corner three by Porzingis, which snapped the big man’s 0-for-5 slump.

Mazzulla kept Tatum in for the first 21 minutes of the second half.

“(He was) locked in defensively,” the coach told reporters. “I enjoy giving him the freedom to play more. He’s getting more minutes, but he’s doing the right thing on both ends of the floor, especially defensively.”

White beat the third-quarter buzzer with a contested, off-balance three between two Heat defenders. The highlight-reel bucket put Boston up 88-65, all but sealing the Celtics’ sixth win in their last seven games. They’ll play their final game before the All-Star break Wednesday night against Victor Wembanyama and the San Antonio Spurs at TD Garden.

Boston has been the NBA’s best road team this season, but it’s struggled on the Garden parquet, going a pedestrian 16-10 in home games and dropping seven of its last 12.

“We give up too many easy ones at home, so we’ve got to make up for it,” Tatum told NBC Sports Boston sideline reporter Abby Chin. “We need to get back on track at home, because we’ve let our fans down too many times this year and sometimes last year. So we owe it to them to play better at home, and we’re going to figure it out.