BOSTON — Ben Simmons sat on the bench with his legs spread wide and his hands clasped between his knees. He was wearing a blue warm-up and holding a clear mouthpiece and following the action with his eyes. This was midway through the fourth quarter of a game that many will tell you was a must-win for the Sixers, yet for the previous eight minutes of action, they had carried on without him Thursday night.

"I'm going to have bad games," Simmons said later, after the Sixers fell behind the Celtics 2-0 in this best-of-seven playoff series with a 108-103 loss. "It happens. Obviously, it's not the perfect timing, but you head home now, we have two home games that we need to take care of and handle business."

Rewind a couple of quarters.

If this 2018 Eastern Conference semifinal does indeed culminate in the end of the Sixers most promising season in more than a decade, we will look back at these as the pivotal minutes: five of them, the last of the second quarter, and an unraveling so sudden and dramatic that you could still feel their impact as the closing seconds of the night arrived. After it was over, and Simmons made the long walk from the visitor's locker room at T.D. Garden wearing a camouflaged jacket and headphones clamping his temples, the fog of questions had yet to lift, each of them iterations of the same essential concept.

What happened?

For the previous two days, there had been talk of adjustments, and pledges that the team whose embarrassing display of defense cost it Game 1 would be replaced by a different unit come Game 2. An hour-and-a-half before game time, Brett Brown spoke in confident tones about his expectations for the evening.

"I feel like you are going to see a different team tonight," the Sixers coach said. "I'll be real surprised if you don't."

And you did. For about 20 minutes of basketball, the team from Monday night seemed dead and gone, another ghost in a city full of them. Three days after making just 5 of 26 attempts from three-point range, the Sixers connected on 7 of 9 attempts over the first 18 minutes, taking a 51-30 lead on a tough Robert Covington jumper from the left corner over tight defense by Terry Rozier.

And then … something happened. The exact nature of that something was tough to gauge given the rapidity with which things unraveled. The team that everybody swore we would no longer see suddenly reappeared between the TD Garden end lines. Over the last five minutes of the first half, the Celtics outscored the Sixers 20-5, with Simmons committing a couple of turnovers in the process.

"I honestly don't know," Joel Embiid said afterward when asked what went wrong. "I have to go back and watch film. They just made shots, they got stops, they were running in transition, and we weren't able to get back."

By the time Jayson Tatum threw down a ferocious tomahawk dunk from the baseline with 2 minutes, 24 seconds remaining in the third quarter, the Celtics held a 76-68 lead and a building that had spent much of the first half flatlining was pulsing with the energy of a fan base fully confident that it was on the side of the better team.

It's hard to argue with that diagnosis, and maybe it was evident even before the Sixers watched their 22-point lead disintegrate. Because the Sixers' identity all season has been Simmons, and from the opening tip he was largely absent. He airmailed a point-blank shot attempt on the game's first possession and later missed bad on a baby hook in the paint. In the third quarter, he got caught with the ball in his hands as the shot clock expired on a dead-end possession.

Late in the third quarter, with his star rookie having attempted just four field goals, none of them successful, Brett Brown called the kid to the bench and let T.J. McConnell take over. When Simmons returned with 5:29 remaining in the game, the Sixers had clawed their way back to take a 93-91 lead. That lead didn't last long.

"I think it was mainly what I did to myself," said Simmons, who finished with just one point on -for-4 shooting with seven assists and five turnovers. "I think mentally, I was just thinking too much, overthinking the plays, and wasn't just out there flowing and playing the way I play, which is free."

There will be plenty of ink spilled between now and Game 3 about what went wrong for the Sixers during their two-game trek to Boston. There are all sorts of theories you can turn to, but the end result is what matters most. Down 2-0, the Sixers find themselves as close to mortality as they've been since before Christmas, when their remarkable run to the No. 3 seed in the Eastern Conference began.

To have a shot at postponing their demise, they need to figure out a way to get their rookie point guard back where he has spent most of the season: in the center of things, dictating to others instead of the reverse.