CLEVELAND — The Celtics entered Saturday night with a .500 record, but their general level of frustration this season has seemed disproportionately high. Stars have struggled, home games have been a chore, and big leads have evaporated.
The Cavaliers have been a plucky upstart over the first month of this season, but no one is confusing them with an Eastern Conference contender. And when the Celtics surged to a 19-point third-quarter lead, the fans here didn’t even seem bothered by it. Their team had a winning record when no one thought it would have a winning record, so maybe they were allotted a dud.
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But then the Celtics eased up long before it was safe to do so. Robert Williams said they “relaxed too much.” Dennis Schröder said they “got stagnant.” Regardless of the phrasing, both would agree that it was not pretty.
In the end, Cleveland pushed back with a 17-0 run and seized an unlikely 91-89 win when Schröder’s baseline jumper at the buzzer was off.
“We just kind of let it get away from us,” Williams said, “because we couldn’t cut that water off when they went on their runs.”
With about one-sixth of this season now complete, it may soon be fair to ask whether Boston’s struggles are the result of being an underachieving group that has yet to find its way, or whether this team is simply just not all that good.
For now, the players remain confident that this is just a blip. Boston was without injured players Jaylen Brown, Al Horford, and Josh Richardson, although Cleveland was missing a pair of starters and a rotation player, too.
One night after erupting for a season-high 38 points in a win over Millwaukee, Schröder poured in 28 points to lead Boston. But he certainly wishes he could have had at least two more.
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With the Celtics trailing by 2 and just under 10 seconds left, coach Ime Udoka drew up a play to give Schröder a chance to drive to the basket against Cavs guard Cedi Osman.
Cleveland briefly delayed the attack with a double-team before Schröder sought an opening on the right baseline. His 13-footer could have sent the Celtics to their fifth overtime game this year. Instead, it caromed off the rim, and several Cavaliers raised their arms in the air.
“I had no choice because they cut me off good,” Schröder said. “They made a good defensive play.”
It was notable that the final play was not called for All-Star forward Jayson Tatum, who scuffled through another tough night and has struggled mightily in the waning moments of games this year. The latest example came Friday, when he missed a potential game-winner at the end of regulation against the Bucks.
On Saturday Tatum was 8 for 22 from the field and 1 for 8 from beyond the 3-point line. His frustration with his own shooting, and with the officials, was apparent throughout the game.
“He has to be able to impact the game when he’s not scoring in the ways he has in the past,” Udoka said, “which he can do.”
The Celtics committed 20 turnovers, eight of which occurred in the messy fourth quarter. It was a sudden shift for a team that had appeared in control for most of the night. Boston held Cleveland scoreless for an eight-minute stretch in the first half and rarely seemed in danger after that.
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A Grant Williams layup with 2:23 left in the third quarter gave the Celtics a comfortable 74-56 lead. But during this topsy-turvy season, comfortable has rarely lasted for long.
The Cavs started their comeback against a bench-heavy unit that included Jabari Parker, Payton Pritchard, Aaron Nesmith, Romeo Langford, and Grant Williams. There is not a true big man or playmaker in that unit, but it put together a nice second-quarter stint so Udoka went back to it in the third, and that’s when the Cavaliers’ run began.
It continued well after Boston’s starters returned. An Evan Mobley dunk with 8:01 left capped the massive 17-0 Cleveland surge that pulled the Cavaliers within 74-73.
The teams mostly volleyed the lead back and forth over the next eight minutes, with neither pushing ahead by more than 2 points.
With less than a minute left the Cavaliers gathered a pair of offensive rebounds before Ricky Rubio found Jarrett Allen for an alley-oop with 35.9 seconds remaining. The Cavs had eight second-chance points in the fourth.
“We had done a great job against one of the top offensive rebounding teams for three quarters,” Udoka said, “and couldn’t come up with the possession late when we had it tied or a slight lead.”
After a timeout, Tatum looked to score quickly to give Boston a two-for-one opportunity. He discarded Osman and calmly hit an open 18-footer to tie the score at 89 with 29.4 seconds left.
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Cavs guard Darius Garland attacked the basket and drew a foul with 9.4 seconds to play. His free throws put Cleveland back in front, and Schroder could not quite answer.
Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.