Jayson Tatum raised an open right hand as he hit the brakes under his own basket and turned his head up to the rim late in Game 2.

He might as well have been holding a white flag.

Cavs guard Caris LeVert had blown by him on a lefty drive Tatum tried to interrupt with a hard foul, only to watch LeVert rise to the basket anyway.

Of course he scored.

Lavert’s layup pushed Cleveland’s lead then to 24, a pathetic punctuation to a pathetic defensive performance by the Celtics in Game 2. LeVert still had free throws to fire and 4:58 froze on the clock, but this game was over.

Joe Mazzulla knew it. He pulled Tatum and the rest of his starters while the TD Garden crowd pushed itself out of every available exit in a slow-moving state of shock.

Unlike last round, this had been a true Game 2 disaster for Boston. A basketball lightning strike — the Heat setting a franchise 3-point shooting record — gifted them one upset before they led for less than a minute combined the rest of the series.

But the Cavs beat the Celtics straight up. They won by 24 and in a way no one could have expected or should have allowed.

Boston, a well-rested, 13.5-point home favorite, got bludgeoned by an inferior team playing its fourth game in seven days. A team missing its starting center, with no viable bench replacement. An opponent they had beaten by 25 points just two days earlier.

This giant pill would be easier to swallow had it been forced down our throats by impossibly hot shooting. It wasn’t.

The Celtics got pounded in the paint, where they gave up 60 points. They got beat on fast breaks and on back-door cuts, with Jrue Holiday losing his man multiple times in the first half and Jaylen Brown all but inviting Isaac Okoro to back-cut him for a score midway through the third quarter, which prompted a Mazzulla timeout.

And would you believe that timeout was Mazzulla’s second of the quarter? The first came after an Okoro transition dunk less than three minutes after halftime.  It felt like a message, an old Ime Udoka “will you stop playing like a——-?”s timeout.

A message about poor effort and focus. About a problem that shouldn’t crop up in the second half of a second-round playoff series, especially when you got gashed for 10 fast-break points in the first half alone.

Then another problem, Cavs guard Donovan Mitchell, predictably exploded. Mitchell scored 23 in the second half, finishing with 29 points, eight assists, seven boards and an unthinkable plus-35 plus-minutes. It was predictable because that’s what players like Mitchell, all-star, All-NBA-caliber, players do.

All of which is to say: why do these Celtics, specifically their stars, need a push to do the same?

Why must it always be some external force driving the Celtics to the internal place — flying around with a fierceness and sharp focus — they know they must live in to win a championship?

Last week, they admitted the memory of past Heat defeats drove them to eliminate Miami. Last year, it took a 3-0 series deficit to drag that dog out of them in the Eastern Conference Finals. That same postseason, they needed a 3-2 deficit to pull the best out of them against Philadelphia, and a Game 5 home upset versus the Hawks to finally oust Atlanta in the first round.

Can’t this team just hand its damn homework in on time?

Their talent is spectacular. Their depth is inarguable. Their injury luck, even as Porzingis sits, continues to be favorable, compared to Miami missing Jimmy Butler and a shallow Cavs roster fighting on without Jarett Allen.

What will it take to finally play tough, consistent playoff basketball?

Because at least Thursday, the Celtics didn’t lose because they were out-coached.

Cleveland’s big early adjustment, opening the second quarter with an all-small, all-shooting bench unit featuring the slow, stocky and short Georges Niang at center, was a disaster. The Celtics hit three straight layups, building to an 11-0 run. Cavs coach JB Bickerstaff called timeout.

Bickerstaff ran the same lineup back out, and Boston kept punking Cleveland, so Bickerstaff called timeout again three minutes later. Then starting big Evan Mobley returned, and Niang was banished until garbage time. The Cavs erased Boston’s lead and forged a 54-54 tie at the break.

In the third quarter, after driving downhill relentlessly, with purpose and force and having reached the bonus after three and a half minutes, the Celtics’ ball-handlers lazed into four 3-point shots on their next five possessions. Just mindless, lazy basketball.

Eventually, Mobley had to take a seat with four fouls before the fourth quarter.  But instead of punishing the Cavs on the inside, where they remain weakest, the Celtics got bullied on the glass by Tristan Thompson — as if the 14-year vet had taken a time machine at halftime — and couldn’t close the gap. Thompson had three boards in six minutes before the benches emptied and drew a foul fighting for another.

Meanwhile, Payton Pritchard, all 6-foot-1 of him, finished tied for the Celtics’ team lead with two offensive rebounds.

After Game 1, Brown opined on the importance of playing hard defense, where basketball focus, effort and toughness manifests.

“That’s gotta be non-negotiable,” he said. We don’t wanna get into trade baskets with teams. We wanna get stops. I think you get stops that allows you to jack up and put up as many threes as you want. And that’s what we wanna do, but we gotta get stops. … So when we do that at a high level, I think we’ll be fine.”

And if they don’t? Well, you saw that Thursday night.

After Game 2, Brown called the Celtics’ defensive performance “unacceptable.” He admitted missing shots on offense affected their defense, as did Mazzulla, as did Tatum.

“It’s the playoffs, that can’t happen,” Brown said.

Except it did.

Again.

But hey, at least now they’re a lock for Game 3.