You heard it from the Sixers for two days:

"This isn't us."

Well, maybe it is.

The Sixers lost to the Celtics, 108-103, and fell into an 0-2 hole in the second round of the Eastern Conference semifinals.

Maybe the Sixers don't have the talent to hang with the Celtics. Worse: Maybe they don't have the moxie.

The team that fought through nasty tactics from the Heat in the first round of the Eastern Conference playoffs has lost its mojo. They dispatched the Heat in five games, and pointed at their inactivity as why they played so badly in the first game of the second round.

The loss in Game 1 was due less to rust than to resilience.

"There is a level of physicality they have apprised to all of us," Sixers coach Brett Brown admitted.

The Sixers didn't have to win Game 2; not necessarily. But they had to let the Celtics know that, no matter the outcome of the series, they wouldn't allow a repeat of Game 1, when they got pushed around. Intimidated. Punked.

Midway through the second quarter, it seemed like they'd served notice. It began with Amir Johnson's hard foul on Terry Rozier, Kyrie Irving's astounding understudy. T.J. McConnell then wrapped up Rozier, and gave him a little shove as they un-clinched. They had turned a one-point lead midway through the first quarter into a 22-point lead 12 minutes later.

Twenty-two points. It was the largest playoff comeback in a decade for the Celtics, who have never lost a series win when they led, 2-0.  The Sixers, conversely,  have never overcome an 0-2 deficit.

How did this happen? Up by 22?

The Sixers just gave it away. The Celtics finished the second quarter with a 21-5 run, and Brown refused to call a timeout. He wanted to save them for the end.

"If I had to do it again, I'd do the same thing," Brown said.

The Sixers never regained momentum. The Celtics won loose balls. They rebounded. They left Ben Simmons open, knowing he wouldn't — couldn't — shoot. They ignored the score.

By the start of the fourth quarter, the Celtics led by four.

As it evaporated, man-mountain center Joel Embiid missed three soft hook shots. More significantly: Embiid  took three soft hook shots.

Simmons, exposed by a team coached to make him beat them, proved again he cannot do that. He was a minus-16 in his first 25 minutes. He scored one point. He missed all four field-goal attempts. He was benched for much of the second half, though Brown reinserted him for the last 5:29. Why?

Part of the Process.

"It's a tough decision. I admit it. This whole playoff experience is something that I want our young guys, our star players, to learn rom, and grow," Brown said. "I'm coming back to Ben Simmons. I think he's going to have to learn how to play in these environments."

It wasn't all Simmons, or Embiid. It was everyone. Well, everyone except JJ Redick, who played great again — 23 points — and McConnell, who ably replaced Simmons for long stretches.

Really, though, the collapse was nearly complete.

With 10 minutes, 40 seconds to play, Aron Baynes denied Embiid another soft layup. Five seconds later, with Embiid still catching his balance and barely inbounds, Jaylen Brown was dunking on the other end.

Amid all of it, Marcus Smart scored 19 points in his second game starting in place of Brown, who returned after a one-game absence and scored 13 off the bench. Dynamic rookie Jayson Tatum, in a continuing nightmare for the Sixers — who traded up with the Celtics to draft bench-bound guard Markelle Fultz — scored 21.

To a man, they played hard, and with purpose, and with heart.

The problem with a lack of heart and a lack of soul is that you cannot scheme those things. You can run screens for shooters and set up big men in the post, but you cannot make a team play hard for 48 minutes. Not even if Meek Mill and Kevin Hart show up.

Embiid picked up his fifth foul with 4:54 to play, which diminished his effectiveness as a rim protector. Twice down the stretch the Celtics went to the rim, and Embiid had to retreat.

Maybe things will change when festivities resume Saturday night in Philadelphia. After all, this is the first playoff run for most of these Sixers, and the first for the franchise since 2012, and the first for Brett Brown as a head coach.

The Celtics have established a level of physicality the Sixers had not yet seen. They'd thought they'd seen physical play against the Heat, but what the Celtics have put on them isn't like the cuff upside Simmons' head from Goran Dragic, or James Johnson plowing over Marco Belinelli and clipping J.J. Redick in the same play. That was dirty. That was cheap.

There's a big difference between being dirty and cheap and physical.

Physical is sustainable. Physical wears you down. Right now, it's wearing the Sixers out. Brett Brown and his players were wrong.

Right now, this is exactly who they are.