Sunday made clear these Celtics are a mediocre team until they prove otherwise

NEW ORLEANS — It’s reached a point where this is expected. The Celtics were coasting in the third quarter, well on their way to a Sunday afternoon, nationally televised, only-NBA-game-on victory over the New Orleans Pelicans and a chance to begin reclaiming their prowess.

Instead, the Pelicans made two simple second-half adjustments and the Celtics were relegated to a junior varsity team just trying to score occasionally against the varsity. Stan Van Gundy completely outcoached Brad Stevens in the second half, and of course, the Celtics blew a 24-point lead and lost, 120-115, in overtime at Smoothie King Center.

Van Gundy made two obvious changes with the Pelicans on the verge of being blown out. He blitzed and trapped Jayson Tatum and Jaylen Brown each time they got the ball near the sideline, and he allowed massive forward Zion Williamson — after the Celtics held him to 4 first-half points — to initiate the offense, to give him a better opportunity to attack the rim.

The Celtics had an answer for neither.

Williamson finished with 24 points and nine rebounds after halftime. The Celtics tried various defenders, but he was too strong for any resistance. Meanwhile, the offense decided to take the rest of the afternoon off.

“This one definitely hurts,” said forward Jaylen Brown, who went 7-for-23 in the game, with 12 points after halftime. “Just a tough loss. We 100 percent should have won that game. I would have liked to play better.”

The Celtics just can’t close teams out consistently. They are 26th (of 30) in fourth-quarter points and 28th in fourth-quarter points allowed. New Orleans, who got trounced, 41-12, Friday in the final period by Phoenix, outscored the Celtics, 34-21.

“We’ve just got to keep playing the game the right way,” Brown said of the fourth-quarter collapses. “In that stretch, we went back to some of our bad habits. We have to match energy and we didn’t, so we ended up losing. We got comfortable and got rolling on their home floor. Then it really becomes hard to try to cut the water off at that point.”

And it was pretty predictable.

“We’ve got a lot of things to clean up,” Stevens said. “But finishing the game is No. 1.”

Boston missed 32 of 48 shots and 13 of 17 3-pointers after the break. The Celtics could have staved off embarrassment and won this game had they hit one or two more shots down the stretch. Tatum saved the team temporarily with a tying floater with 0.2 seconds left in regulation, but it never should have come to that point.

The Celtics actually led, 84-63, with 4:17 left in the third quarter, and then allowed the Pelicans to go on a 13-3 run, with Williamson scoring three consecutive buckets. The offense then went into panic mode, Kemba Walker and others jacking up shots with no success.

Walker and Jaylen Brown combined to miss 32 shots and 18 3-pointers. That’s not Stevens’ fault, but the Celtics never made the necessary adjustments, and then Tatum tried saving the day with his old habit of settling for stepback fadeaway jumpers. (Eight of Tatum’s 14 misses were on them.)

But he got caught into hero ball because he wasn’t getting any help offensively. The pressure was mounting as New Orleans was making its fourth-quarter surge and the Celtics were left helpless.

This game was a collective effort of mishaps. Stevens could never put his players in positions to succeed or to combat a simple trap defense, and the players just failed to make shots. The Pelicans offense became predictable in the second half, and why shouldn’t it have been? The Celtics had no one to check Williamson, and that resulted in either layups or fouls.

What is even more infuriating is the consequence of doubling or even tripling Williamson was never discovered because the Pelicans were awful from the 3-point line after halftime, 4-for-20 from beyond the arc. And just the Celtics luck, on the most pivotal play of the game, Brown helped out on Williamson even through Robert Williams was the second defender waiting for him.

That left Brandon Ingram open for the back-breaking 3-pointer in overtime. The one time the Celtics overcompensate to stop Williamson, they’re burned.

“We can put anybody on our team in front of Zion one-on-one and he’s going to get where he wants to go,” Stevens said. “We’re going to try to challenge it as well as we can. That’s why you have to guard him as a team. Sometimes you help, you get burned.”

Mediocre teams find ways to lose, or not to win. The Celtics are a mediocre team, not some team that had underachieved. They don’t deserve that title. This is who they are until they change it.

Boston nearly blew a 27-point lead Friday against Atlanta. This debacle was on a bigger stage, with the nation watching and now understanding this team’s dysfunction.

“This was a tough loss, but I think we played exceptionally well for a large part of the time,” Brown said. “We’ve just got to mature and grow up. We were really moving it and getting guys involved. Just trying to get other guys involved. I think that’s better for our team.

“We’ve got to come out and find ways to win and we dropped the ball [Sunday].”

Gary Washburn can be reached at gary.washburn@globe.com. Follow him on Twitter @GwashburnGlobe.