The Boston Celtics expected to have their entire roster available Monday night for just the second time this season.

Minutes before tipoff, plans changed.

Jayson Tatum, two nights removed from his sensational 43-point triple-double against Chicago, was added to Boston’s injury report with what the team called a non-COVID illness. That was at 6:58 p.m. ET, ahead of a 7 p.m. start in Orlando, Fla.

Tatum was pulled from the starting lineup, did not join his teammates on the bench and eventually was ruled out at halftime, forcing the Celtics to make do without their team leader in points, rebounds, assists and steals.

For 30 minutes, it looked like they wouldn’t need him. Boston led by 15 points at halftime against the injury-ravaged Magic, who have been playing without stars Paolo Banchero and Franz Wagner (torn obliques) and lost sixth man Moritz Wagner to a torn ACL on Saturday.

The Tatum-less C’s couldn’t hold it, however. Orlando dominated the second half, building a double-digit lead and then surviving a late Celtics rally to secure a physical 108-104 win at the Kia Center.

Jaylen Brown shined in his co-star’s absence, finishing with 35 points, nine rebounds, four assists and three steals to lead the Celtics. But Boston set season lows in points, (104), assists (13), 3-point attempts (32) and 3-point makes (eight) and nearly matched its season high in turnovers (19) against an Orlando team that, despite its litany of ailments, boasts one of the NBA’s top-ranked defenses.

Brown, Jrue Holiday (16 points) and Derrick White (17 points) each turned the ball over five times. Kristaps Porzingis went 13 for 14 from the foul line but just 2 for 10 from the floor and 0 for 4 from 3 in the loss.

The Celtics fell to 22-7 heading into its Christmas Day matchup with the Philadelphia 76ers at TD Garden.

With Tatum in the locker room, Brown spearheaded the Celtics’ offense during a terrific first quarter. Boston’s co-headliner scored 15 points on 7-of-11 shooting, added four rebounds and two steals, and was the driving force behind a surprisingly dominant interior game.

Orlando entered Monday allowing just 44.1 points in the paint per game, the second-best mark in the NBA behind Oklahoma City. That’s typically not the Celtics’ offensive focus — they came in ranked last in the league in paint points scored per game — but they piled up 24 in the first quarter alone, with Brown providing half of those. Boston led 32-21 after one.

The Celtics’ success attacking the rim helped them withstand another less-than-stellar showing from beyond the arc. The NBA’s most productive and prolific 3-point shooting team went 3 for 15 from deep in the first half Monday and made just one 3 in the second quarter (by Al Horford at the 5:08 mark).

Despite that lack of long-range firepower, Boston kept Orlando at arm’s reach throughout the second and took a 15-point lead into halftime. It added another 12 paint points, including made rapid-fire layups by Brown and Holiday sandwiched around a stolen inbounds pass. Brown, Holiday and White all reached double figures in the first half.

Stout defense helped, too. The Celtics held the shorthanded Magic to 43 first-half points on 33.3% shooting (20.0% from 3) while blocking five shots and notching seven steals.

Momentum then shifted drastically after halftime. Orlando scored the first seven points of the second half to trigger a quick Joe Mazzulla timeout, then reeled off runs of 9-1 and 10-1 later in the third quarter. The latter tied the game at 69-69.

An acrobatic reverse layup by Cole Anthony around Luke Kornet and Kristaps Porzingis gave the Magic their first lead of the game with 2:10 remaining in the third. Anthony scored 27 second-half points two nights earlier to help Orlando overcome a 22-point fourth-quarter deficit and beat Miami.

The Celtics and Magic entered the fourth quarter tied at 79-79. Orlando quickly stretched its lead to seven points, but Boston climbed back to within one possession after Brown drew a foul on a dunk attempt and Holiday hit a 3. The Celtics couldn’t close the gap, however, and 35 seconds later, the Magic were back up seven.

Boston then proceeded to score on four straight possessions — including three consecutive Brown field goals — and five of six. But the Magic matched each make with one of their own. A 3-pointer by Kentavious Caldwell-Pope made it 100-93 with 5:12 remaining. Another by rookie Tristan Da Silva a minute later turned it into a 10-point game.

An already rough-and-tumble game then turned chippy when Porzingis clashed with Orlando’s Goga Bitadze and Jalen Suggs during a coach’s challenge. The altercation resulted in technical fouls for Porzingis and Suggs and an ejection for Bitadze, who grabbed Porzingis by the throat while the Boston big man was jawing with Suggs.

After that tussle, the Celtics surged. Brown hit a floater. White drilled a 3, then won possession out of a jump ball after a Magic miss. Holiday hit a triple to trim Orlando’s lead to three, and Brown put back his own offensive rebound to make it 105-104.

But that’s where Boston’s comeback stalled. Da Silva buried a 3-pointer with 9.2 seconds remaining, and Brown’s attempted answer rang iron.

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