Consider the advantage of Joel Embiid’s injury wasted after Celtics defensive debacle

“Al is one of our best defenders,” Mazzulla said. “[Harden] hit a big shot.”

Philly wins Game 1, 119-115, without Joel Embiid and the Celtics again have made matters difficult on themselves. This one’s on Joe Mazzulla because he got outfoxed by Doc Rivers, who tried everything to slow down the Celtics, including a zone defense that hindered several possessions.

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While the offense was struggling to close out the game, the defense couldn’t contain Harden and his Hardenelles, the Philadelphia supporting cast who took turns gashing a lackadaisical Boston defense. The Celtics did a good job of stopping exactly nobody in Game 1, after which Horford admitted the team may have lost focus when Embiid was scratched about 90 minutes before the game with a sprained knee.

The Celtics traded baskets with the 76ers instead of focusing on stops. Harden tied his career playoff high, set eight years ago with the Houston Rockets. The only way for the 76ers to steal Game 1 was for Harden to transform back to Houston Harden, and the Celtics just let it happen.

The point of late-game defense is disallowing the opposing team’s star player or most prolific scorer from attempting their desired shots. If you’ve watched NBA basketball for the last decade, you’ve noticed Harden likes to dribble into a rhythm 3-pointer.

It’s not that the Celtics lost this game, it’s that they lost this game because of their stubbornness. They tested a man who averaged nearly 37 points per game five years ago, and who is third all-time in 3-pointers, to shoot with an aging forward defending him.

“The last three or four minutes, we couldn’t get stops,” guard Malcolm Brogdon said. “We were scoring, staying ahead by maybe 2 or 3 points, but we weren’t able to get stops. And for a team like that, that doesn’t have Embiid, they want to keep the game close so a guy like James Harden can close it out, which he did.

“We didn’t get stops all night long and that’s really the answer. We played good initial defense and then had some type of breakdown for a layup. And I don’t think we came out in the second half the right way. Came out and played a little sloppy.”

The Celtics don’t fare well with prosperity. On April 25, they played to the level of the Hawks, who were without second-leading scorer Dejounte Murray because of a suspension, and fell apart in the final five minutes as Young got hot.

In the final 2 minutes, 28 seconds Monday, the Celtics shot 1 for 7 with two turnovers, including a direct pass from a panicking Brogdon to Philadelphia’s Tyrese Maxey, who scored on a breakaway layup in the final 30 seconds.

“I believe, it happened to us last year when one of their main players was missing and Embiid being out tonight, as much as you don’t want to say it wasn’t a factor, I think it was,” Horford said. “And we have to be better. We have to understand we have to be able to play with whatever happens, whatever variables. Without our team, I’ve just seen it happen for whatever reason.”

The series is far from over, but the Celtics lost perhaps the most winnable game. They fully knew Philadelphia would play small, spray the ball out to 3-point shooters, and allow Harden to be ball dominant to score or get to the free-throw line.

They saw it coming, but either couldn’t do anything about it or didn’t care to.

“A lot of stuff didn’t work, as we’ve seen,” forward Jaylen Brown said. “They just came out more prepared than we did. They came out and played their asses off and they have a great game and we had a mediocre game, and we lost.”

Rivers coached a great game. Mazzulla coached a mediocre one. Robert Williams, who was a plus-8 (as opposed to Horford’s minus-17), didn’t play enough minutes in crunch time. The Celtics didn’t make enough defensive adjustments and they were stymied by the 76ers zone defense in the second half.

Rivers used his array of tricks and wrinkles to baffle Mazzulla and it worked. Mazzulla needs to be better for the rest of this series. He needs to avoid being so predictable and, perhaps when it matters, rely on double teams and other defenses besides switching because it’s burned the Celtics twice in the past week.

The Celtics will have to respond in Game 2, a must-win because they wasted whatever advantage they had with Embiid ailing. Again, they relied on the tried and tested when that plan doesn’t necessarily work against elite coaches and elite players. Mazzulla has to counter with some wrinkles and tricks of his own.

Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him on Twitter @GwashburnGlobe.