The Boston Celtics let a winnable game slip away to the Philadelphia 76ers on opening night. The Celtics’ defense and late-game execution both disappeared in crunch time at TD Garden, a sign the team has a lot to sort out as they embark on a season of the unknown.

Figuring out how to play — and win — without Jayson Tatum is going to be a process. The Celtics got a good learning experience on opening night, but blew a winnable contest to a subpar team. These are the opportunities the Celtics cannot let slip away as the season progresses.

The Celtics trailed at halftime on Wednesday night but took control in the third quarter by outscoring the 76ers, 35-18, in the frame. It still looked like the C’s would escape with a win as they led by nine points with 3:34 left in regulation, but then it all fell apart.

Philadelphia ripped off a 15-2 run over the final three minutes, and the Celtics missed two game-winning looks in the final seconds to drop their opener, 117-116, in disappointing fashion. The Celtics need to be a scrappy and opportunistic team this season, and they want to be a squad known for their defense. Not much of that applied Wednesday night.

Allowing 42 points in the fourth quarter isn’t going to get it done. And when the game was on the line, it was Payton Pritchard who got the game-winning looks — twice — over Jaylen Brown.

We went into the season with a lot more questions than answers, and we have even more questions after a tough loss to the 76ers. Here are some of the main takeaways from Boston’s season-opening defeat.

Celtics couldn’t close out 76ers, figure out last shot

The Celtics had no answer for Tyrese Maxey (40 points) or rookie VJ Edgecombe (34 points in a historic debut) throughout the game. Both had big fourth quarters too, as Maxey dropped 15 points while Edgecombe put up nine. Boston’s perimeter defense was non-existent in the fourth, as Philadelphia shot 7-for-10 from downtown in the final frame. It was a Kelly Oubre three with 1:12 left, which put Philly on top 112-111, which really change the final minute of the contest.

But the Celtics still had a chance to steal the win back in the final seconds after Edgecombe missed a pair of free throws. Boston had two looks at a buzzer beater, but Pritchard missed on both of them.

Pritchard’s driving layup attempt with 3.3 seconds left was rejected, but the Celtics got the ball back after it went out of bounds on Oubre. After he got the inbound pass from Derrick White, Pritchard missed again from 13 feet out at the buzzer.

That Jaylen Brown didn’t get a single look at the end of the game is mystifying. White said the final play was to get the inbound to Brown, but the Philadelphia defense shut it down.

Pritchard had done work to get Boston back in the game in the closing minute. He hit a three with 15.9 seconds left and then a driving layup with 9.1 on the clock to make it a one-point game. The reigning Sixth Man of the Year, who had 16 points off 6-for-14 shooting, said he made the wrong read on his first game-winning attempt.

“I thought I could get [Quentin Grimes] in the air and draw a foul there, but the right read was a little kick out to Anfernee [Simons],” Pritchard admitted. “So, it’s a growing process. Learn from it, you get better. So, on to the next.”

Brown, who had 25 points off 9-for-18 shooting, didn’t even touch the ball on those final two possessions. The Celtics have a lot to figure without Tatum in the lineup, and making the most of those end-of-game possessions is near the top of the list. 

Brown, who looked a bit off after a hot start, didn’t seem worried about the end-of-game pecking order.

“I’m looking forward to watching film and learning and staying with the journey,” Brown said after the loss. “We got a new group that’s still figuring things out. Some of these other teams have played together for a little bit more, so there’s going to be some ups and downs. But I’m looking forward to just learning every day with the group. And I’m looking forward to when we get to that point where we catch our wind, and what that could possibly be.”

Boston’s defense struggled

Allowing a 42-point fourth quarter completely doomed the Celtics. For a team that wants to be known for their defense, that simply cannot happen.

The Celtics did shut down Joel Embiid (four points) and anyone not named Maxey and Edgecombe (allowing just 43 points to eight other 76ers). But again, they let Maxey go off for 40 and Edgecombe drop 34 in his first-ever NBA game. 

“I don’t know if we were too focused on the offensive end, but we can’t let guys just come in here and do what they want,” Brown said postgame.

Joe Mazzulla said transition defense was a major issue for the Celtics after the loss. Boston allowed 21 fast-break points on Wednesday night.

The Celtics are really going to have to up their defense going forward, and it has to start Friday night when they pay a visit to the New York Knicks. 

Three-ball wasn’t falling for Boston

The Celtics are still going to shoot a lot of threes. It helps when they make those threes, but the long-distance strike wasn’t falling Wednesday night. 

The Celtics shot just 11-for-43 from downtown (25.6%) against Philadelphia. They went just 2-for-7 in the fourth quarter.

Pritchard was just 1-for-7. White had 25 points, but shot just 4-for-13 from three. Anfernee Simons went 2-for-6 off the bench, but Chris Boucher, Xavier Tillman, and Luka Garza shot a combined 0-for-7 from deep. 

The 76ers were the better three-point team on Wednesday night, as they hit 16 of their 40 attempts (40%) overall and went 7-for-10 from downtown in the fourth quarter. 

In Queta we trust

We’ll end on a positive, which was big man Neemias Queta for Boston. He got the start and finished with 17 points (off 7-for-8 shooting) and eight rebounds over his 25 minutes of action. He came up huge in the fourth with eight points on a perfect 4-for-4 shooting and three rebounds before he fouled out with 1:42 left.Â