Marcus Smart and the Celtics struggled with turnovers and it led to them losing Game 1 of their second-round playoff series against the 76ers on Monday night. Charles Krupa/Associated Press

For as long as this series lasts, it will be a simple test against the Sixers.

The Celtics can survive volcanic James Harden eruptions, like his 45-point outburst Monday night.

They can navigate a no-show from Derrick White, who scored four points and sat down the stretch.

They can even endure the return of expected MVP Joel Embiid, who lost three of four regular-season matchups and has yet to beat Boston when it matters most.

But what the Celtics cannot, will not and did not survive Monday is losing the ball.

Boston lost Monday’s 119-115 thriller when Malcolm Brogdon gifted a go-ahead, fast-break layup to Tyrese Maxey at 28.9 seconds to play, and Marcus Smart’s pass for Jayson Tatum instead found Paul Reed with less than 10 seconds left. The Celtics finished with 16 turnovers. Philly had six.

That was it. That was the game.

Yes, between Smart’s dump of a dump-off and Brogdon’s brain fart, Tatum hit two foul shots to push the Celtics ahead 115-114.

Tatum had curled right after receiving an inbounds pass, chased contact close to the basket and drew a whistle. It was one of the only times the Celtics played with any sense of purpose and organization in the final 2:30, an aimless stretch defined by two turnovers and a panicky 1-for-6 shooting performance.

Meanwhile, Harden calmly orchestrated the Celtics’ collapse with master stroke after master stroke. After Tatum’s free throws at 26 seconds left, he dragged Al Horford into a pick-and-roll Boston switched atop the key. He drew Horford tight with a couple of low dribbles, and triggered a game-winning 3-pointer faster than the big man could close.

“He probably surprised me with how quickly he shot the ball,” Horford admitted later.

Harden’s pick-and-roll mastery was nothing new to the Celtics, who two possessions earlier had successfully danced around several screens he had beckoned for. But the last one, as the clock ticked under a minute to go, sprung Reed free for a rim attack, and Smart fouled him in the act. Reed hit both foul shots.

Leading 113-112, the Celtics pinged the ball around on their penultimate half-court possession. Five seconds left.

Tick. Smart up-faked on a 3, dribbled to the left elbow and flipped a pass back to Tatum.

Tock. Tatum pump-faked a 3 and refused to shoot.

Tick. Now, it was Brogdon’s turn to panic, and he vomited a pass to Maxey who coasted three-quarters of the court in disbelief until his layup fell through, and the crowd fell silent.

A second epic collapse had been thrust upon TD Garden in a matter of 24 hours. How could this happen?

The truth is these were not cruel, lingering ghosts from the Bruins’ Game 7 loss. Not even bad luck. The Sixers, Daryl Morey’s Sixers, won with math. The possession game.

They took 14 more shots than Boston and fired a dozen more 3-pointers.

“We only shot 26 threes,” Celtics Coach Joe Mazzulla said. “I don’t know if that’s the best way to go about it.”

Philly claimed 32.4% of its available offensive rebounds – a much higher percentage than the Celtics’ – and generated the rest of their extra shots off turnovers. That effort, even without Embiid, was enough to overcome a Boston team favored by 10 points that shot 58.7% from the floor. Fifty-eight percent!

But whether they shoot 58% or 48% or 38% in Game 2, the Celtics have their solution against Philly. Aside from protecting the ball – Smart was the worst offender with six turnovers – Boston must hit the gas and pound the paint.

The Sixers without Embiid have little rim protection. With or without Embiid, they’re slow. Their transition defense is atrocious, allowing opponents to score 1.18 points per possession, fifth-worst in the league. Evidence of how pace and a focus on the paint could hurt Philly was ample early in Game 1.

The Celtics’ first six points were two dunks and a layup. Midway through the first quarter, Tatum cooked Tobias Harris, one of the Sixers’ better defenders against him, off the bounce for a layup. A minute later, Jaylen Brown back-cut Harden and cocked a two-handed slam behind his head for a thunderous dunk.

The Celtics led by seven after the first quarter, when 26 of their 38 points came inside the paint. They scored 30 of their 66 first-half points on possessions that lasted fewer than 10 seconds. Good shots found the Celtics when they did the hard work early, instead of trying to work Philly in the half-court.

Mazzulla also has to smooth out wrinkles in his rotation. Grant Williams entered with 5:10 left in the first quarter and was promptly hunted off the floor. Harden beat him for two baskets around a Harris blow-by. With 1:30 left in the quarter, the Celtics sent a double team to help Williams against Harden, and all that did was generate a wide-open Georges Niang 3-pointer.

Splash. Williams finished with four minutes played.

Mazzulla later threw Sam Hauser into the fire. He saw 33 more seconds  than Williams and finished with a matching zero points.

Of course, White must play better. Rob Williams could see a longer leash considering his rim protection, which might allow the Celtics’ perimeter defender to play higher out on Harden. Time will tell, as it usually does.

The tenets of a Game 1 are rarely the truths of a long series — except maybe this one. The Celtics lost the ball Monday night. They lost the game and their home-court advantage. Protect your possessions, or the series will follow.

On to Game 2.