The Sixers were dreadful in their series opener. Here’s how they plan to fix it for Game 2.

CAMBRIDGE, Mass. — Nick Nurse typically asks his 76ers video staff to edit game footage into offense, defense, and “special teams” sections that can succinctly “tell the story” of what unfolded.

Not so with his team’s brutal 123-91 Game 1 loss to the Celtics. Players and coaches rewatched the game in its entirety on Monday afternoon, with plenty of stops and starts along the way.

“It was pretty painful,” Nurse said following the Sixers’ practice at Harvard. “ … We did a lot of things not very well. A lot of things out of character. So it was long. Lots to look at and lots to talk about.”

Painful yet necessary, veteran center Andre Drummond added.

» READ MORE: The Sixers’ shooting was a big problem in Game 1. The bigger problem is it might not have mattered.

By methodically dismantling the Sixers on both ends of the floor, the Celtics in Game 1 looked the part of an NBA Finals contender. And the Sixers will remain shorthanded for Tuesday’s Game 2, as former MVP Joel Embiid has begun a strength and conditioning program in Philly following his April 9 emergency appendectomy.

Yet the Sixers still believe there are elements they can improve to make this a more competitive first-round series.

“Physically watching what we did to kind of beat ourselves,” Drummond said of Monday’s film session, “the things that we could have done better and things we could have done more of. … Yes, it was very long. Very detailed. We stop and go. A lot of things we wrote on the board for what we can do [Tuesday].”

The Sixers’ 4-of-23 shooting mark from three-point range on Sunday was horrendous. So a possible solution is to … generate more of those beyond-the-arc attempts?

Nurse thought that, after the Sixers’ initial flurry of misfires from deep, “everybody decided they were going to pump-fake and drive into the lane.” In the third quarter, the Sixers attempted only four three-pointers.

“We just got a little gun-shy,” Nurse said. “Which is too bad, because I thought the other night [in the Play-In Tournament win against the Orlando Magic], I thought we played so free. It was almost too free, but it’s better than pump-faking and trying to drive everything when you have open threes.”

All-Star point guard Tyrese Maxey — who connected on 36.7% of his 8.6 three-point attempts per game during the regular season but is still hindered by a finger injury — said after Sunday’s loss that he needed to create more opportunities for himself to launch from deep. And he encouraged the same when he kicks out to teammates such as Kelly Oubre Jr. and VJ Edgecombe, who both went 0-for-5 from three-point range in Game 1.

“If you miss four, you miss five, you’ve got to shoot the next one anyway,” Maxey said. “ … If you miss a few, you get a little nervous. But I’m shooting, no matter what, if I’m open. I want everybody to shoot.”

That let-it-fly mentality is particularly important because Boston’s 16-of-44 mark from deep on Sunday was in line with its average regular-season output (15.5 makes per game, at 36.7%). That means the Celtics are still capable of catching fire later in this series … or having an uncharacteristically cold night.

Even with the parade of missed shots, the Sixers were perhaps most disappointed in their listless defensive effort.

The Sixers allowed 20 fastbreak points, which were often sparked by one of their 15 turnovers. And the Celtics too often got to the rim via “walk-down-the-lane” finishes, prompting Maxey to say they simply need to step in front of a ballhandler when they get free. Especially when that ballhandler is Jayson Tatum or Jaylen Brown, two of the league’s premier scorers.

“Those are the ones we can’t give up,” Maxey said. “It’s going to be tough, regardless. And we’re capable of doing it, but we can’t give those types of plays up.”

After Monday’s practice, reserve guard Quentin Grimes attributed those blunders to too many miscommunications and not “being on the same page for 48 minutes.”

Nurse upped his rotation to nine players during Sunday’s real minutes. In the first three quarters, those reserves — Drummond, Grimes, Justin Edwards, and Dominick Barlow — combined to shoot 3-of-10 from the floor for nine points.

Zero of those shot attempts during that stretch came from Grimes, one of the league’s better sixth men and a needed scoring spark while Embiid is sidelined. When asked Monday about the lack of shots, Grimes said, “It just be like that sometimes. Kind of how the game flows.”

» READ MORE: Sixers defense offers ‘no resistance’ in Game 1 blowout loss to Celtics and ‘you’re not going to win like that’

“We’ve just got to do a better job of getting him in spots where the ball can find him,” Nurse added. “That’s as simple as that.”

After Edwards did not play in Wednesday’s game against the Magic, he provided seven points and six rebounds in his playoff debut. Drummond, like teammate Adem Bona, picked up two quick fouls. And Barlow, who normally thrives as a cutter and offensive rebounder, went 1-for-6 from the floor and added three rebounds in 15 minutes.

The Sixers also struggled to finish uncontested opportunities at the rim Sunday — and to avoid unforced errors.

One sequence that embodied those squandered opportunities came in the middle of the second quarter, when Bona missed two consecutive putback attempts. About a minute later, Oubre could not connect on a driving layup and Edwards missed a tip-in attempt.

“The rim was a big factor on both ends,” Nurse said after the game.

The Sixers cut the Celtics’ lead to 15 points in the third quarter on an Oubre tip finish, then forced a shot-clock violation. But Oubre missed a three-pointer, and then Drummond threw a bad pass intended for Maxey off a rebound. That ignited a 10-0 Boston run to increase the lead to 25 points, and the Celtics never looked back.

“We just never could get one [bucket] to go to give us maybe one more spark,” Nurse said.