HOUSTON — For much of the night, the atmosphere at the Toyota Center was considerably muted. It was quiet enough that a young fan sitting about 40 feet from Jayson Tatum was able to get his attention as he sat on the bench, earning a friendly wave for the effort.
It felt like a group of fans watching a 15-win team and not really expecting much. But the Celtics seemed to view the situation similarly.
“You think it’s just going to automatically happen, and that’s not the NBA and it’s not basketball,” Jaylen Brown said. “And we didn’t have a sense of urgency, so we weren’t rewarded tonight. That’s what happens.”
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What happened was a humbling 111-109 loss against a Rockets team that entered the night with the worst record in the NBA.
The Celtics spent large segments of the game appearing either worn down or simply disinterested, as if they would be able to win when they decided it was time. In their defense, they almost did, clawing back from a 9-point deficit with less than three minutes left.
But on the game’s final play, Tatum sliced to the rim and missed a last-second layup that would have forced overtime. There had been a margin for error, but Boston misjudged its size.
“We kind of played with the game and tried to win at the end,” Tatum said, “instead of playing how we’re supposed to play the entire game from start to finish.”
Boston is just 5-5 since returning from the All-Star break, with this loss the latest puzzling setback. The Celtics (47-22) remain in second place in the East, but they are now tied with the 76ers (45-22) in the loss column.
Last season, Boston battered one opponent after another following the All-Star break and appeared fearsome when the postseason began. There’s still time for a late surge, but it has at least been delayed.
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“I would rather [the struggles] happen now than later,” Marcus Smart said. “I’d rather it happen now than when we get into the playoffs and it happens.”
Brown poured in 43 points on 16 of 25 shooting, and Tatum had 22 points but was 2 for 10 from the 3-point line, with 4 turnovers. Fellow starters Al Horford, Smart, and Derrick White combined for just 17 points.
The Celtics made just 12 of 42 3-pointers and had much more success getting to the rim against a young, sometimes physically overmatched Rockets team. Boston made 60 percent of their 2-point attempts.
But coach Joe Mazzulla, who has been an ardent defender of his team’s long-range approach, said this game was not lost because of poor outside shooting. Several times he pointed out how the Rockets had more second-chance points (17-10), free throws (27-21), and offensive rebounds (15-10) — and fewer turnovers (11-14).
“That’s the game,” Mazzulla said. “That’s everything. Those margins are extremely important. So, I don’t care if you shoot half-court shots. if you don’t win those margins you’re not going to win many basketball games.”
Many of those numbers are directly related to hustle and effort. For the Celtics, that probably arrived too late.
“We picked it up kind of too late,” Tatum said. “And I take the blame for that. I know I didn’t necessarily start the best and I feel like that kind of spread throughout the team.”
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Jalen Green scored 28 points to lead Houston.
“We knew Boston wants to get as many wins as they can, and we kind of wanted to spoil that,” the Rockets’ Kevin Porter Jr. said.
The Celtics trailed by as many as 13 points in the third quarter but began to restore order with strong drives to the rim, with Tatum leading the charge. A Brown 3-pointer with 10:32 left in the fourth pulled the Celtics within 87-85, and they appeared in position to avoid disaster.
Then the Rockets drilled three 3-pointers in a row, with big man Usman Garuba hitting two. A Porter Jr. runner with 4:42 left gave Houston a 104-93 lead. Houston still led by 9 with fewer than three minutes left when the Celtics rallied.
A 3-point play by Malcolm Brogdon with 1:44 left pulled Boston within 106-104, and the Rockets were called for a foul on a box-out on Brogdon’s free-throw attempt, and Smart’s ensuing free throw made it a one-point game.
A Green pull-up stretched the lead back to 108-105 before Brown was fouled on a 3-pointer with 52.3 seconds left. Brown missed the first free throw before hitting the next two.
Kenyon Martin Jr. missed a 3-pointer but chased down his own rebound, a glaring issue in this game and for much of this season for Boston.
“We’re just not committed to it right now, for whatever reason,” Mazzulla said. “And we have to get back to that.”
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With about 21 seconds left to play and 3 seconds on the shot clock, Green kicked the ball away near the sideline. As he fell out of bounds Tatum saved it behind his back, but the Rockets recovered the loose ball and avoided a shot-clock violation. The Celtics were forced to foul.
“I should probably call a timeout,” Tatum said. “But I guess they felt like I had possession and when I tried to save it, and obviously the shot-clock went away. Situations like that where if you don’t play the right way from the beginning that sometimes things like that come back to bite you.”
Green hit both free throws, giving the Rockets a 110-107 lead with 10 seconds left. Brown attacked quickly for a layup with 8.7 seconds to play before Jabari Smith Jr. was fouled with 7.8 seconds left, but hit just one of two free throws.
After a timeout, Tatum had a clear lane to the basket, but missed the contested layup.
“We didn’t deserve to win today,” Brown said.
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.