Before Game 1 of this Eastern Conference semifinal series Monday night, 76ers coach Doc Rivers looked gloomy when he talked about having to face the Celtics without the injured presumptive league MVP, Joel Embiid.
But Rivers said his team had a “kitchen sink mentality” when Embiid is absent, and he was hopeful it would be impactful for at least one night. The junk defenses Philadelphia threw at Boston caused some issues, but at the other end, James Harden caused even bigger ones.
Thrust into a larger scoring role with Embiid out with a knee sprain, Harden turned back the clock and erupted for 45 points in his team’s stunning 119-115 win.
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“I haven’t felt one of those zones in a minute,” Harden said, “and it felt really good.”
Harden’s 3-pointer with 8.4 seconds left flipped a 1-point deficit into a 2-point lead, and on the Celtics’ next possession Marcus Smart turned the ball over looking for Jayson Tatum under the hoop.
Just like that, 24 hours after Boston fans trudged to the TD Garden exits after the Bruins’ season came to a shocking end with their Game 7 loss to the Panthers, the team colors changed, but the general mood returned.
Monday’s game, of course, marked the start of a series rather than an ending that punted a team into summer. But the loss will certainly be enough to make fans jittery considering how the past two days have unfolded.
Now, the 76ers will have no reason to rush back Embiid for Wednesday’s Game 2, partly because they have already wrestled home-court advantage from the Celtics, and partly because they just won on this floor without him.
The Celtics knew for days that Embiid was unlikely to play and insisted his absence would not cause them to ease up. But after Game 1, Al Horford acknowledged that perhaps it had.
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“As much as you want to say it wasn’t a factor, I think it was,” he said. “We have to be better.”
At the start of the night, however, it appeared the Celtics got a jolt of confidence from Embiid being sidelined. They attacked the rim willingly and relentlessly, and mostly got layups on arrival, clearly pleased to not see Embiid’s massive 7-foot-1-inch frame lurking.
The Celtics started 17 for 20 from the field, a blistering stretch that would usually be enough for a substantial lead. But it staked them to just a 7-point cushion.
They connected on 27 shots over the final three quarters, with the 76ers using a zone defense that did just enough to disturb Boston’s rhythm. Celtics coach Joe Mazzulla said his team started to pass up open looks in search of even better ones, and that led to forced shots and turnovers.
“I think the zone changed the game for us,” Rivers said.
The Celtics’ runway to the rim may also have given them a false sense of comfort. This team has been built around 3-point shooting, with Mazzulla imploring his players to launch them until they can’t launch any more.
They attempted 26 on Monday, their lowest total this season and second lowest of the last two years.
“I don’t think 26 threes is a recipe for our success as a team,” Mazzulla said.
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Regardless, the Celtics’ main issue was their defense. A unit that soared toward the top of the league by season’s end was gashed during much of the surprisingly challenging first-round win over the Hawks.
On Monday it allowed a team playing without the NBA’s leading scorer to shoot 50.6 percent from the field and 44.7 percent from the 3-point line, with Harden calmly leading the charge.
“There were several times I thought, they were just scoring on us,” Rivers said. “And we said, ‘We’re going to have to score back tonight.’ "
Tatum had 39 points to lead the Celtics, but he was just 1 for 4 in the fourth quarter. Jaylen Brown’s disappearance was even more glaring. After a 6 for 7 opening quarter, he attempted just three shots over the rest of the game.
Mazzulla said he needs to do more to keep Brown (23 points) involved, and Brown said he’s not sure what led to the lack of opportunities. With Boston leading by 4 with just over two minutes left, Brown appeared to have an excellent fast-break chance, but decided to pull the ball out instead.
“You always want to make sure your guys are running with you,” Brown said. “And right there I just felt I was running by myself.”
The Celtics were clinging to a 114-113 lead with under a minute left when several players passed up decent looks before Malcolm Brogdon drove with the shot clock running down. He then fired a pass toward the top of the key and it went right to Tyrese Maxey as the clock expired.
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Players from both teams hesitated, assuming a violation would be called. As Maxey raced upcourt, he even looked over his shoulder toward a referee before taking a layup, as if even he was unsure it would count.
But the basket was good. After Tatum put the Celtics back in front with a pair of free throws, Harden took care of the rest, lulling Horford with a few crossover dribbles at the top of the key before pouring in the game-winner.
“He probably surprised me how quick he shot the ball,” Horford said. “I figured he would have tried to spend a little more time before going. But I felt like I was right there.”
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.