The most demoralizing factor in the Celtics’ loss in Game 5 of the Eastern Conference semifinals — a game Boston led by 14 points in the fourth quarter — is they didn’t play hard enough when it counted.
The Bucks didn’t use a dominant fourth-quarter by Giannis Antetokounmpo to steal this game. They won Wednesday at TD Garden because they outhustled their opponent, chased down offensive rebounds, swiped to cause loose balls and nit-picked the Celtics into submission.
The 110-107 Celtic defeat was one of the worst here in recent memory for a team that played stellar for 3½ quarters, then looked inept in the final minutes. And the defending champion Bucks showed championship mettle, including Antetokounmpo, who splashed a crucial 3-pointer — off an offensive rebound — that started the Celtics’ collapse with 1 minute, 40 seconds left.
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He finished third in the MVP voting earlier in the day but Antetokounmpo played like an MVP when it counted. He was 3 of 20 from the 3-point line before he launched that long ball with the Bucks trailing by 3 in the final two minutes. He released it without hesitation and it was nothing but net.
The Celtics responded by settling. A Jayson Tatum contested jumper. A Marcus Smart turnover when he tried dribbling in traffic. Another clutch 3-pointer by Jrue Holiday, tying the game.
Mentally, the Celtics just looked frazzled. After Antetokounmpo missed a potential tying free throw with 14.2 seconds left, Bobby Portis squeezed in between Smart and Jaylen Brown and scored the go-ahead put-back.
Smart then returned to his hero-ball ways by driving the lane and practically handing the ball to Holiday, who blocked his shot, grabbed the ball and banged it out of bounds off Smart. Then the Bucks called timeout.
It ruined what was a masterful performance by the Celtics, who for about 41 minutes were the better team in this series. But all that hard work and execution was wasted when they proved futile in the final seven minutes. The Celtics scored 8 points in the final 6:48 as the Bucks seemingly converted every key shot, and if they missed, they certainly gathered the offensive rebound.
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They snatched 17 offensive rebounds, Milwaukee’s most since Dec. 4. Portis grabbed seven, making up for an off shooting night by outworking the Celtics in the paint. It was embarrassing.
“It was about pride,” guard Marcus Smart said. “And they out-prided us with those 17 offensive rebounds. If we box out, we win the game. They’re the defending champs, they made some championship plays and now we have to respond.”
And now they are on the brink of elimination, a sparkling season at risk unless they respond and win Game 6 at Milwaukee. Although the Celtics have been resilient this season, always seemingly responding from tough losses with big wins, this may have been the toughest. They had the Bucks staggering, played with more energy and passion but obviously forgot that old boxing term that you have to knock out the champion to beat the champion.
The Celtics tried stalling the final rounds for a split decision. They got TKO’d in the final seconds.
This is the worst time for harsh lessons. The Celtics have proven throughout this series they are better than the Bucks, until the key moments. They rallied to take the late lead in Game 3 and then missed consecutive open 3-pointers that would have sealed the game. Antetokounmpo and Holiday clinched that game with buckets in the last minute.
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The Celtics have lacked such heroics in this series, those closing plays. There are going to be games where you have to win ugly, where a steal, blocked shot or yes, offensive rebound, makes the difference. The Celtics made the mistake of letting up when they should have followed through, getting too cute with possessions or settling for contested shots instead of attacking the rim.
Boston was in the bonus for the final 8 minutes, 1 second of the fourth quarter. That should have resulted in several easy points. Instead, the Celtics attempted four free throws in that span, all from Jayson Tatum.
Playoff games are far too valuable to just give away. This was a swing game in this series. The Celtics had won two of the past three and lost in the final seconds of Game 3. They had all the momentum and backed it up in Game 5 with stellar basketball until they succumbed to the fortitude of a defending champion.
Similar to those losses to the Miami Heat in the NBA bubble, the Celtics had this game taken away by a hungrier team, a team that never relented because you can’t relent in playoff games. This isn’t the second of a January back-to-back when fatigue and complacency sets in.
These are when the games count, when every possession is crucial, every loose ball and rebound needs to be pursued with vigor. The Celtics forgot that for the final six minutes, and well, now they are one loss from being done.
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“Guys are going to be pissed about the outcome; we outplayed them for 3½ quarters,” Celtics coach Ime Udoka said. “We talked about showing our resolve. We made it tougher on ourselves now but it will be sweeter when we bounce back. But we know we gave up a golden opportunity tonight.”
It will be disheartening if the Celtics have to point to this one game, this six-minute breakdown as why their season ended prematurely, why they were good enough to reach the NBA Finals but they fell two rounds short because they forgot the principles that put them on the verge of winning this series.
There is a stain that will never be erased at playoff losses like this — just ask some of those aging NBA legends who can breakdown plays from a playoff defeat 40 years ago. The only way the Celtics can make up for this failure is by responding with a vengeance Friday in Milwaukee. But do they have enough determination left to bring this series back to Boston? That’s debatable.
Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him on Twitter @GwashburnGlobe.