It was unclear until about an hour before tipoff Monday night if Jayson Tatum would play against the Miami Heat.

The Boston Celtics superstar’ was added to the injury report on Sunday and listed as questionable with a knee issue. With the All-Star break looming, Boston could have opted to give its most important player an extra day 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 a sloppy start and cruised to a 103-85 win at the Kaseya Center.

The Celtics were missing starters Jrue Holiday (shoulder) and Jaylen Brown (knee), but replacements Al Horford and Sam Hauser went a combined 9 for 16 on 3-point attempts. 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 the franchise-reshaping Jimmy Butler trade.

Miami’s 85 points were tied for the second-fewest by a Celtics opponent this season.

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 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, 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.

Boston held Miami scoreless for nearly seven 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 lacked his usual long-range effectiveness during the first half of the season because of a lingering back injury, but he looks more like his sharp-shooting self of late. He’s shot better than 50% on 3s in nine of his last 10 games, inching his season-long shooting 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 tend to be 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.

Hauser helped the Celtics erase their 11-point deficit. They led by as many as 16 before halftime, outscoring the Heat 39-12 during that game-changing stretch.

Another core reserve, Payton Pritchard, played the final seven minutes of the first quarter and the entire second. Though he went just 1 for 5 from beyond the arc, he had five points, six assists, six rebounds and two steals in the first half. He finished with 10 points and eight assists in 36 minutes.

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 3 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 third quarter in Boston’s blowout win over the New York Knicks at Madison Square Garden. In this one, Tatum hit three 3s, sank an elbow jumper and went 4 for 4 on shots at the rim, tearing apart Miami’s new-look lineup. He also grabbed four rebounds and assisted on a corner 3 by Porzingis, which snapped the big man’s 0 for 5 slump.

White beat the third-quarter buzzer with a contested, off-balance 3 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.