What do you get when you combine the Celtics’ thinnest roster of the season with one of Jayson Tatum’s shakiest showings of 2024-25?

Evidently, an easy Boston victory.

Down two starters and two of its top reserves, the Celtics blew out a similarly shorthanded Miami Heat squad 108-89 on Monday night at TD Garden.

Jaylen Brown led Boston with 29 points on 8-of-21 shooting, with Payton Pritchard delivering another stellar performance off the bench (25 points on 10-of-17, including 5-of-12 from 3-point range). Derrick White added 19 points, eight assists and six rebounds in his return from a foot injury, and Luke Kornet tied his career high with six blocked shots.

Tatum had 18 points and 11 rebounds and committed a season-high six turnovers.

Miami, which was playing without All-Star Jimmy Butler and five other players, shot 35.6% from the floor and 22.9% from three and trailed by double digits for most of the second half.

Kristaps Porzingis and Al Horford sat out on the second night of a back-to-back, with the previous night’s injury issues forcing head coach Joe Mazzulla to play both in a road loss to Cleveland rather than staggering their games off. Jrue Holiday also was ruled out with a knee injury, and Sam Hauser was a late scratch for personal reasons, leaving Boston without four of its core rotation players.

Tatum, Brown and White all were listed as questionable before getting the green light, with Brown and White returning after missing the Cavaliers game. Mazzulla called Brown (illness) a game-time decision, and his availability was not confirmed until after his pregame warmup.

From that group, Mazzulla assembled a brand-new starting lineup, plucking Kornet and Jordan Walsh off the bench to join Tatum, Brown and White. It was just the second career start for Walsh, a 2024 second-round draft pick who was coming off back-to-back healthy DNPs and hadn’t played more than eight minutes in any game since Nov. 8.

The Celtics fell into an early 7-0 hole but reeled off a 17-2 run to surge ahead. Kornet fueled that push with three makes at the rim, including one off a White alley-oop.

Boston is the NBA’s most prolific 3-point shooting team, but its first made three didn’t come until the 5:44 mark of the first quarter, after Mazzulla had subbed in an almost entirely new second unit that surrounded Tatum with Pritchard, Neemias Queta, Xavier Tillman and Drew Peterson.

Peterson is a two-way player who’d seen minimal NBA playing time before his surprise 25-minute shift Sunday night. Tillman has struggled to crack the rotation since the opening week of the season.

Tatum hit two threes after the line change but went just 3-for-10 from the floor in the first quarter. The Celtics took a slim 28-25 lead into the second.

That advantage then spiked thanks to Pritchard and Brown, who combined to score Boston’s next 14 points. Two threes from White and one from Peterson helped the Celtics stretch their lead to 17 points, and it sat at 60-45 at halftime.

The second quarter also featured three Kornet blocks and a cameo from Baylor Scheierman — the first meaningful minutes of the first-round draft pick’s young career. Scheierman, who’s been developing in the G League, notched a rebound, an assist and a steal and was a plus-7, though he looked like he’s still adjusting to the speed of the NBA game.

The Celtics slogged through an ugly start to the second half that included Tatum turnovers on three consecutive possessions and a 7-0 Miami run. The Heat got to within eight before Boston recovered, holding the visitors to one made field goal over the next seven minutes. By the time that slump ended, the rout was on, with a Brown 3-pointer putting the Celtics up 78-57 with 3:05 remaining in the third.

Another of Pritchard’s patented buzzer-beaters — this one a slick driving layup rather than a long-range three — extended Boston’s lead to 22 entering the final quarter. The Celtics outscored the Heat 25-18 in the third despite turning the ball over nine times.

This was the fourth straight 20-point performance for Pritchard, who continues to strengthen his case for NBA Sixth Man of the Year.

Peterson, who spent time with Miami’s G League affiliate as an undrafted rookie last season, stared down the Heat bench after hitting a fourth-quarter 3-pointer. The 6-foot-9 wing showed potential in his fill-in stints this week, finishing Monday’s game with seven points on 2-of-5 shooting and seven rebounds.

Mazzulla emptied what was left of his bench in the final minutes, inserting JD Davison and Jaden Springer with the victory secured.

The Celtics will host the Detroit Pistons on Wednesday before playing another back-to-back against Milwaukee and Memphis on Friday and Saturday. They’ll also find out their NBA Cup fate on Tuesday, as the outcomes of Knicks-Magic and Bucks-Pistons will determine whether Boston advances to the knockout rounds of the in-season tournament.