BOSTON — The Celtics’ 2025-26 season ended Saturday the way it began: with a home loss to the Philadelphia 76ers that Jayson Tatum watched from the bench.

With Tatum, whose early-March return from Achilles surgery was supposed to elevate Boston from impressive overachievers to true title contenders, a late scratch due to left knee tightness, the Celtics dropped Game 7 at TD Garden, 109-100.

Boston erased a 15-point first-half deficit and nearly climbed out of an 18-point hole in the fourth quarter, but a drought of 10 consecutive missed field goals — eight of them from 3-point range — doomed its comeback bid.

The Sixers advanced to the Eastern Conference semifinals, where they will meet the New York Knicks. It was the first time in Celtics history — and just the 14th time in NBA history — that they lost a best-of-seven series after leading 3-1.

The season-ending three-game losing streak was Boston’s first since October, when it started 0-3 before emerging as one of the league’s most formidable teams. The Celtics entered the postseason as the No. 2 seed in the East and the betting favorite to reach the NBA Finals. Instead, they bowed out in the opening round for the first time in five years and just the second time in the last decade.

After taking a 13-point lead early in the second half of Game 5, the Celtics were outscored by 51 points over the rest of the series.

Jaylen Brown finished Game 7 with 33 points, nine rebounds, four assists and three blocks. Derrick White (26 points, 5-for-16 shooting from 3, three blocks) and Neemias Queta (17 points, 12 rebounds) played their best games of what was an underwhelming series for both players, who struggled with inefficient shooting and foul trouble, respectively. Top trade-deadline acquisition Nikola Vucevic was a healthy DNP.

Joel Embiid, whose return from an appendectomy in Game 4 changed the trajectory of the series, put up 34 points, 12 rebounds and six assists for the Sixers while racking up double-digit drawn fouls. Tyrese Maxey had 30 points on 11-of-18 shooting, 11 rebounds and seven assists.

Rather than plug one player into Tatum’s spot in the startling lineup — Baylor Scheierman and Jordan Walsh would have been the two most natural candidates for that, based on how coach Joe Mazzulla managed his roster in the past — the Celtics coach chose chaos. He replaced not one, but three starters, booting out Queta and wing Sam Hauser and inserting Scheierman, Luka Garza and Ron Harper Jr.

Scheierman and Garza both had seen sporadic playing time over the first six games — the latter as the third-choice center behind Queta and Vucevic — but Harper had only played in blowouts. He was one of the final additions to Boston’s 15-man roster, only having his two-way contract converted to a standard deal last month.

The new-look starting group of those three, plus mainstays Brown and White, had never played together before Saturday.

Mazzulla’s goal, it seemed, was to recapture the dynamism Boston showed in the fourth quarter of Game 6, when the Celtics nearly halved a 23-point Sixers lead after pulling their starters with 10-plus minutes remaining. Scheierman, Garza and Harper all were part of the bench mob that staged that comeback. They also starred in Boston’s stunning Game 82 upset of Orlando, combining for 84 points as a collection of eight Celtics reserves beat the full-strength Magic in the regular-season finale.

Saturday’s results from that bunch were less impressive. The Celtics fell behind 11-3 before Mazzulla made his first sub (Payton Pritchard in for Harper) and trailed 32-19 after one quarter. Scheierman, Garza and Harper all finished the game with zero points.

Boston’s biggest issue during its inauspicious start? A lack of resistance in the lane. The Sixers owned a 22-6 edge in points in the paint during the opening period, which allowed them to build a double-digit lead despite misfiring on eight of their first 10 3-pointers.

But the Celtics recovered. As the second quarter opened, Mazzulla called Brown to the bench and inserted Hugo Gonzalez, who’d been out of the rotation for weeks and had logged just six garbage-time minutes in his first NBA postseason. The high-energy rookie immediately changed the tenor of the game with his defense on Maxey, VJ Edgecombe and Embiid.

That, coupled with much-improved play from White and Queta on the offensive end, powered a furious Celtics rally. Boston opened the second quarter on an 18-4 run, with White and Queta providing 13 of those points. The other five came from Pritchard, whose 3-pointer put Boston in front, 37-36, with 8:51 to play in the first half. The entire run occurred with Brown on the bench.

The Celtics shot 5 for 7 from 3-point range in the second quarter. White hit two, and Pritchard, Brown and Hauser made one apiece. The Sixers regained their composure late in the half, with Embiid putting both Gonzalez and Queta in foul trouble and then rejecting a Brown dunk with 30 seconds remaining. But the C’s entered halftime within striking distance, down 55-50.

Mazzulla utilized a more conventional lineup to open the second half, and Queta immediately drove for a layup. He was whistled for his fourth foul shortly thereafter, however — a backcourt foul after a Sixers rebound, of which he committed six in the series — halting both his and the Celtics’ momentum. The 76ers ripped off an 8-0 run that made it 63-54.

The Celtics shot 4 for 17 from 3 in the third quarter, including an 0-for-4 effort from White, who was limited to two points in the quarter after scoring 19 in the first half. Philly led 88-75 entering the fourth.

Boston then opened the final quarter the same way it did the second, playing with renewed enthusiasm as it trimmed the Sixers’ cushion from 18 points to one.

Brown scored on a cutting layup from White to make it 90-84 with 9:24 remaining and trigger a Philadelphia timeout. Then, he drew both a Paul George foul and an Embiid technical foul on a hard drive to the basket. After Queta converted a tough and-one hook shot over George, Brown hit a midrange jumper through contact for another three-point play.

A White 3-pointer cut it to 95-94 with 5:52 to play. Queta picked up his fifth foul on the ensuing possession, and Mazzulla, after unsuccessfully challenging the call, kept the big man in.

The Celtics proceeded to miss four 3s that would have tied the game or given them the lead — two by White, one by Brown and one by Pritchard, who couldn’t convert an open look from the corner after Brown blocked George at the rim. Boston played nearly the entire fourth quarter with those three, Hauser and Queta on the floor.

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