TORONTO — When it was really possible, when the shorthanded Celtics really had a chance to beat the streaking Toronto Raptors, who were at full strength, Boston couldn’t make a winning play at the end.
Despite a valiant effort, and a 4-point lead with less than a minute left in regulation, the Celtics never made that final play. Without the injured Jayson Tatum and Jaylen Brown, the Celtics’ offense sputtered in key moments and the Raptors dominated the offensive glass in Boston’s 115-112 loss Monday night at Scotiabank Arena.
Without four starters, the Celtics traded blows with their rivals for 53 minutes, with coach Ime Udoka digging deep into his bench for support. But after Marcus Smart scored a layup with 3:16 left in overtime for a 110-106 lead, the Celtics did not record another field goal. They relentlessly tried attacking the rim but the tall, athletic Raptors allowed nothing in the paint.
On the final possession, Smart and Grant Williams missed open looks on 3-pointers, until Fred VanVleet secured the rebound in the final seconds. The Celtics fell out of first place in the Eastern Conference and had their six-game winning streak snapped.
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“Hell of a job fighting on the second of a back-to-back shorthanded,” Udoka said. “But there are a lot of things we left on the table, kind of shot ourselves in the foot. Love the fight, loved the effort but we had our chances.”
The Celtics entered Monday heavy underdogs but actually held the lead most of the fourth quarter, including 101-94 with less than four minutes left before VanVleet changed the game with a pair of contested 3-pointers.
Still, Boston had a two-possession lead with 58.4 left in regulation after a difficult corner 3 from Aaron Nesmith, who has experienced a difficult second season.
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Pascal Siakam, who led the Raptors with a season-high 40 points, cut the deficit to 2 with a quick layup, but the Celtics couldn’t muster a quality look on their final two possessions.
Tied with 12.6 left, Grant Williams was stripped by VanVleet, forcing an already tired team into overtime.
Missing two players who averaged a combined 50.5 points per game, the Celtics offense expectedly sputtered against one of the league’s top defenses. Smart and Derrick White tried attacking the rim late in regulation and overtime but the Raptors packed in the paint and contested every shot.
Smart led the Celtics with 28 points on 10-for-25 shooting but he missed nine 3-pointers. Williams added 17 while White scored 15 and Nesmith added 13, playing his first significant minutes since coming back from an ankle injury.
Despite the difficult loss, Smart refused to look at the result as negative. The Celtics put together a makeshift roster Monday, including a pair of G-League call-ups.
“It was tough, especially the road we’ve been on, the competitive nature that we all endure inside of us but we’re not looking at it like [a defeat],” he said. “Obviously we wanted to win but we understand we’re shorthanded, played [Sunday] night, traveled, called guys up. They don’t know what we’ve been doing all year.
“It was a tough game, so the fact we did what we did and had chances, I’m proud of us.”
Siakam, who has struggled against the Celtics, hit back-to-back buckets to give the Raptors the lead for good in overtime. The Celtics seemingly had defensive answers for every other Raptor option, but Udoka was forced to used undersized Nesmith in the post against the Toronto big man.
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With Tatum and Brown home resting, Robert Williams out indefinitely with a left meniscus tear, and Al Horford dealing with personal matters, the Celtics played valiantly. But 18 turnovers along with some key missed free throws ruined a chance for an improbable win.
Still, the Celtics’ defensive blueprint was apparent despite the injury to Williams. The Raptors shot 37 percent and the team missed 29 3-point attempts.
“It was a great night by a lot of guys, a lot of guys stepped up,” Udoka said. “We let one slip away. Like I said to the group, the effort wasn’t the problem, it was the execution. We’ve established who we are and regardless of who’s out there, we’re going to play that way.”
The Celtics defense was solid all night, but they gave up steals and easy layups at an alarming rate, especially in the second period. A key play occurred at the end of the first half, when the Celtics had a chance to close out the quarter with a two-possession lead. VanVleet instead stripped Payton Pritchard near midcourt and scored a layup and free throw with 3.3 seconds left.
Despite being so shorthanded, the Celtics made a statement early, playing one of their more masterful quarters of the season, piling up 38 points in the opening period on 56 percent shooting. Udoka called on former G-Leaguers and two-way guys to help out, giving Sam Hauser and Luke Kornet quality minutes, in addition to starting little-used Nesmith.
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The result was hustle, ball movement and sparkling 3-point shooting. The Raptors staggered for a while, as the Celtics were the better team despite having three former All-Stars watching from home.
The opening quarter was a distinct message that the Celtics wouldn’t be pushovers, regardless of how much this game meant to Toronto, which is fighting the Cleveland Cavaliers to avoid the play-in tournament.
The win kept the Raptors one game ahead of the Cavaliers for sixth, but the Celtics walked away fully confident about a potential playoff series with their full squad.
Gary Washburn is a Globe columnist. He can be reached at gary.washburn@globe.com. Follow him @GwashburnGlobe.