CLEVELAND — It’s hard to tell whether the Boston Celtics were due to underperform in a big game or if old demons have yet to be exorcised.

Perhaps they were looking ahead to a showdown and possible Finals preview with the champion Denver Nuggets, or maybe they relaxed just a bit against the pesky Cleveland Cavaliers, who were without Donovan Mitchell and later, Evan Mobley.

The Celtics still own the league’s best record by a country mile, and still look every bit as formidable as many expect they will when the playoffs begin next month. The duality of the Celtics was on full display, blowing a 22-point lead in the fourth quarter and losing 105-104 to the Cavaliers after a puzzling final possession Tuesday night, which snapped Boston's 11-game winning streak.

Dean Wade not only outscored the Celtics in the fourth quarter by himself (20-17), he scored the clinching basket, a follow-up dunk from a Darius Garland miss with 19 seconds left that sent the RocketMortgage Fieldhouse crowd off on a high it didn’t expect.

On the Celtics' final possession, Jayson Tatum, as he’s often wont to do in late situations, got the switch he wanted against Garland, and instead of going early to give his team multiple chances at taking the lead, he took a fadeaway and missed. He was almost bailed out by a foul call on Garland, but officials reviewed it and said Tatum's leg extended unnaturally and created contact with Garland, reversing a trip to the free-throw line with .7 seconds left and effectively ending the game.

It was slightly unfamiliar territory for the Celtics, who blew out the Golden State Warriors by 52 points on Sunday and outscored their opponents by 22 during this streak which began Feb. 4.

Celtics coach Joe Mazzulla said he didn’t mind Tatum getting the look he had with his height advantage over Garland. But Jarrett Allen was coming to Garland’s aid to change Tatum’s shot at the last second, and even if Tatum got the look he wanted it didn’t look easy.

“I thought we were very composed to be in that situation. Composed is one thing, executing is another,” Mazzulla said. “Execution isn’t about the results, half the possessions we got really good looks. The other ones, we gotta get better shots.”

If the circumstances of a player like Wade getting hot or Tatum just not going sooner was enough to grant the Celtics a one-time “get-out-of-jail-free” card, Celtics All-Star Jaylen Brown was having none of it.

He probably took a look at the box score, where Cleveland shot 73 percent in the fourth to the Celtics' 31 percent.

“Today matters,” said Brown, who scored 21. “Whether everybody wants to throw it away or not, we gotta look at the film and address some stuff because that matters. Your habits are everything, your mentality is everything. You can’t waste possessions or time out there on the floor.”

Brown is a more serious type, so he would take a loss like this harder than someone who’d view it as one game in 82. He’s been around long enough with this franchise to know the scrutiny that’s ahead.

Especially with Mitchell out with a bruised left knee and Mobley leaving after a right ankle sprain, the Celtics were almost in a no-win situation. Winning is what was expected, by virtue of the streak and the upcoming showdown in Denver.

Boston has Finals expectations and enough playoff heartbreaks to count, so this loss isn’t befitting of a serious team that treats opponents with appropriate fear. But the Celtics still sport the league’s best defense and most explosive 3-point shooting, a fact that can’t be obscured by this bad loss.

In today’s NBA, shooting the three and defending it are arguably the two biggest principles that lead to winning, and the Celtics shot 42 percent from 3 during the streak while holding opponents to just 32 percent.

They make their free throws (sixth in percentage), they’re one of the best rebounding teams in the league and their defenders all seem malleable, switchable and hardly ever out of position.

The sky isn’t falling and they’re still on pace to easily eclipse 60 wins if they don’t take their foot off the gas after they inevitably clinch home court throughout the playoffs.

The Cavaliers coming back from a fourth-quarter deficit seems fitting for a team that looks better when key parts are missing and reserves are thrust into main roles, but the Celtics seem like they can ill afford their high-performing players missing a beat.

Tatum was a ghastly 1-of-9 in the fourth and the Celtics missed all eight of their 3-point attempts while Wade was perfect overall and hit five triples — being deployed as a small-ball center, keeping the floor spread and pace somewhat frenetic. An anomaly of epic proportions.

“It’s not in our guys’ nature to give up. We’ve been shorthanded, we’ve been down,” Cavaliers coach J.B. Bickerstaff said. “It seems every single time it happens, somebody finds a way to step up and help us win. The momentum got going, the crowd got going, shots started falling.”

That’s when the veteran leadership of the Celtics is expected to step in and calm things. The lessons from the meltdowns and disappointments should be the scar tissue that keeps matters from capsizing.

“For the most part we had some good [looks]. We could get some better ones,” Mazzulla said. “Seventeen fourth-quarter points, you know they’re gonna make plays.”

And if Tatum’s call goes the other way, it seems likely he makes the free throws and the Celtics escape without having to learn a lesson in the loss column. But the weaknesses everyone can point out came to bear, and it’s enough to cast a little doubt moving forward.

“We played well enough for a good portion of it. In the second half, fourth quarter, they got hot, we didn’t, they got in a rhythm,” Tatum said. “It played out, it was unfortunate. I thought I was fouled, which was tough because we would’ve had a tip-in. A weird way to end the game, but they always say a game isn’t won or lost in the last play. A lot of things that went wrong in that fourth quarter.”

A lot of things have gone right for this team so far this season. We just won’t know if Tuesday is a harbinger of things to come, or a mere blip on the road to June.