The Celtics erased a 24-point first-quarter deficit Tuesday night in Toronto but couldn’t complete the comeback, falling to the Raptors 119-118 to close out their preseason schedule.

Boston, which went 4-1 in its tuneup games, now will turn its eyes toward opening night. The reigning NBA champions will open the regular season next Tuesday, Oct. 22, against the New York Knicks at TD Garden.

Here are six takeaways from Tuesday’s loss:

1. Starters go deep

This was dress rehearsal night for the Celtics, with head coach Joe Mazzulla giving his top dogs their longest run of the preseason.

Boston utilized an eight-man rotation for most of the game, not dipping deeper into its bench until halfway through the fourth quarter.

Payton Pritchard, Derrick White, Jaylen Brown, Jayson Tatum and Al Horford started for the Celtics and all played at least 23 minutes. Luke Kornet, Xavier Tillman and Jordan Walsh were the top three reserve options, with Neemias Queta also making one second-quarter cameo with the regulars.

Starting guard Jrue Holiday and bench shooter Sam Hauser both did not dress for the game.

Tatum led a balanced Celtics scoring effort with 24 points. White added 23, Brown 19 and Pritchard 15. Tatum, White and Pritchard attempted a whopping 38 3-pointers in the game and made 15 of them (39.5%).

Boston’s benchwarmers kept things competitive late, with JD Davison hitting a go-ahead three with 26.1 seconds remaining. But Davison fouled D.J. Carton on the ensuing Raptors possession, and Carton’s two made free throws proved to be the difference.

2. An unfamiliar opening

After bulldozing through their previous three games, the Celtics quickly found themselves down double digits to a sweet-shooting Raptors squad.

Toronto’s Scottie Barnes and Gradey Dick combined for 20 points in the first quarter alone, hitting seven of their first eight 3-pointers. Toronto as a team shot 71.4% from deep in the first and led by as many as 24.

The Celtics, meanwhile, went 2-for-12 from three over the first 12 minutes.

An unsustainable disparity? You bet.

Down 46-27 after one quarter, Boston proceeded to score the first 19 points of the second, buoyed by three triples from preseason MVP Pritchard. Tatum and Walsh also hit threes during that Celtics scoring blitz, with Kornet adding a pair of layups.

The Raptors did not make a single 3-pointer in the second quarter (0-for-13) and Boston took a 67-66 lead into halftime.

3. Tatum drops the hammer

The highlight of Boston’s ugly first quarter was the thunderous dunk that Tatum threw down over a helpless Bruno Fernando.

Seconds later, Tatum stole an errant Toronto pass and took off for another dunk in transition, prompting a Raptors timeout. His back-to-back slams gave the Celtics a much-needed momentum boost, and their 19-0 run began shortly thereafter.

Tatum’s numbers over his 18 first-half minutes: 6-for-12, 3-for-6 from three, 15 points, six rebounds, four assists, two steals, plus-18.

Barring a regular-season regression, the Celtics’ centerpiece seems to have overcome the shooting problems that plagued him last postseason and at the Olympics. Tatum shot better than 37% from three in all four of his preseason appearances.

4. Al Horford debuts

Horford took the floor for the first time this preseason after sitting out the first four games for rest purposes. The 38-year-old big man showed some rust in his return, going 1-for-6 from the floor and 0-for-4 from three in his 23 minutes. He finished with two points, two rebounds and one assist.

Mazzulla cycled through multiple frontcourt combinations, playing Horford with both Kornet and Tillman for stretches. How he deploys that trio and Queta will be a key early-season storyline while the Celtics await the return of Kristaps Porzingis.

Horford spoke highly of his three understudies in a pregame interview with NBC Sports Boston’s Abby Chin.

“I’ve been really impressed just with the improvement with them,” he said. “… It’s just exciting when you see those guys really take another step. Even though it’s the preseason, you can already tell that they’ve gotten better.”

5. Long look for Jordan Walsh

At this point, it would be surprising if Walsh didn’t have at least a minor role with Boston this season. The 20-year-old won’t always see as much playing time as he did Tuesday night — his 24 minutes were inflated by Holiday and especially Hauser sitting out — but he showed more this preseason than any of the other candidates for the rotation spot previously occupied by Oshae Brissett.

Walsh’s tough layup with less than a minute remaining tied the game at 115-115. He tried for another as time expired but couldn’t convert that one over 7-footer Branden Carlson. He finished with seven points, three rebounds, one assist and one steal.

At the very least, Walsh shouldn’t find himself buried in Maine as an NBA sophomore. His improvement after a forgettable rookie year was one of the more encouraging developments of Celtics camp.

6. No minutes for Lonnie Walker IV

Speaking of being buried in Maine, the G League looks like the most likely landing spot for Walker after Mazzulla chose not to play him in the preseason finale.

The NBA-proven wing looked good during both of Boston’s warmup games at TD Garden, but his odds of landing the team’s final open 15-man roster spot always were slim thanks to the Celtics’ precarious luxury tax situation. (Signing Walker, even for the veteran minimum, would trigger close to $10 million in tax penalties.)

The deadline for teams to set their regular-season rosters is 5 p.m. ET on Oct. 21, so we’ll know Walker’s fate by then.

Guard Jaden Springer, who has a guaranteed $4 million salary for this season, also did not see the floor Tuesday night.

Philly injury scare

One non-Celtics injury situation to monitor: 76ers All-Star newcomer Paul George left Philadelphia’s preseason game in Atlanta on Monday with a hyperextended left knee.

George was diagnosed with a bone bruise and suffered no structural damage, per ESPN’s Shams Charania, a relief for Philly. But durability has been a major issue for the 34-year-old. Before playing in 74 games for the Clippers last season, George missed at least 26 in each of the previous four campaigns. It’s unclear whether this latest injury will sideline him into the regular season.

Center Joel Embiid also has a troublesome injury history, with the Sixers choosing to sit the 2023 NBA MVP for the entire preseason to rest his knee. The ability to keep their stars on the court will be the No. 1 X-factor for a 76ers team that, on paper, has one of the most talented rosters in the Eastern Conference after signing George this offseason.

This season’s first Celtics-Sixers matchup is on Christmas Day at TD Garden.

Off the rim

The Celtics announced details for next Tuesday’s banner-raising and ring presentation festivities. Fans are encouraged to be in their seats by 6:45 p.m., with the ceremony scheduled to begin at 7 p.m. Tipoff for Celtics-Knicks will follow at 7:30 on TNT. … The Celtics are, unsurprisingly, the preseason favorites to win their second straight NBA title. BetMGM Sportsbook had their championship odds pegged at +325 as of Tuesday, well ahead of the runner-up Oklahoma City Thunder (+650). Tatum also is BetMGM’s NBA MVP favorite at +800, with OKC’s Shai Gilgeous-Alexander second at +900. … ESPN rolled out Part 1 of its “NBA Rank” series on Tuesday, covering the players ranked No. 51-100 entering this season. Horford checked in at No. 96, with ex-Celtic Marcus Smart ranking 83rd. Expect all five of Boston’s top-choice starters to land somewhere in the top 50.