It shouldn’t have come down to that possession at the end of regulation.

Phoenix had this one in the bag.

The Suns led by 17 points with seven minutes left, but they allowed Boston to come back and put itself position to tie the game and force overtime with 6.1 seconds left.

Then, Marcus Morris, a former Sun, did just that in knocking down an open 3 from the top of the arc with .03 seconds left off a pick-and-pop with Kyrie Irving.

Five minutes later, the chant of “LET’S GO CELTICS” rained through Talking Stick Resort Arena to signal more than just a 116-109 Boston win.

The Suns gave this one away.

“We just got comfortable,” Suns guard Devin Booker said. “Instead of putting a team away, we gave them life.”

Booker scored a season-high 38 points, but Kyrie Irving went for 39 as Boston, which had a horrid first quarter, handed the Suns an excruciating loss.

“A bitter taste in our mouth,” Suns coach Igor Kokoskov said.

Kokoskov said it was an option to foul Irving, but took responsibility as the head coach.  

“We leave the huddle, we have to know the coverage,” the first-year NBA head coach said. “The coverage was to block any ball screen, which is switching and also any catch that far out from the 3-point line is to foul.”

Three Suns — Booker, Isaiah Canaan and Deandre Ayton — all said the call was to switch.

Booker: “We were supposed to switch everything. Just a miscommunication.”

Canaan: “We were supposed to switch it at the end.”

Ayton: “I was guarding (Jason) Tatum. We’re switching on everything.”

 

On an inbound play, Tatum threw the ball to Morris, who ran from the baseline to above the 3-point line. Booker, who was guarding Jaylen Brown, switched and picked up Morris after Brown screened TJ Warren.

Morris handed the ball to Irving, who ran from the other side of the half-court line. Irving dribbled left. Booker switched, but Canaan tried to stay with Irving and left Morris free. 

Irving pitched it back in traffic between Booker and Canaan to Morris, who hit his second 3 of the quarter as Trevor Ariza rushed to contest the game-tying shot.

“Coach drew up a great play,” Morris said. “He figured they would probably go with (Irving) because he carried us in that fourth quarter.”

Unfortunately for Phoenix, Stevens figured right.

“You get caught up in the game,” Canaan said. “Sometimes you get over aggressive thinking that the best player on the team is going to take the shot and he passed it. (Morris) made the shot.”

Ayton called it a “simple mistake,” but it led to a crushing defeat.

“That’s what got us,” he said.

Kokoskov said he believes in fouling in end-of-game situations, but added it’s tough against the “very sneaky” Irving, especially when considering how he said the referees were calling shooting fouls.

“You’ve got to trust your guys,” Kokoskov added. “The call was foul.”  

Phoenix didn’t foul, or switch, as Morris rose up from his sweet spot and nailed it to tie the game, force overtime and send the Suns reeling.

“Shocked,” said Ayton when asked the team reaction to Morris’ 3. “Shocked.”

Booker tried to put the Suns on his back and traded buckets with Irving. Booker scored seven of the Suns’ first nine points in overtime while Irving scored the Celtics’ first six points.

Then, Jaylen Brown hit his only 3 of the game with 2 minutes 5 seconds left to break a 109-109 tie and begin a 7-0 Boston run to close out a game the Celtics had no business winning.

“You could tell they got tight,” Morris said. “Devin Booker played a good game and he did well as he’s been doing but you could tell. They got a little sticky right there.”

The Suns should’ve won this game for two reasons:

They started great — and Boston didn’t.

Warren came out on fire and showed no signs of the back spasms that hindered him in Tuesday’s 22-point loss to the Brooklyn Nets, where he failed to score.

Warren finished with a season-high 29 points Thursday night.

“My team did a good job of finding me in my spots and just being myself out there and staying aggressive and try to stay efficient on both ends of the floor,” Warren said.

Ayton was more aggressive early, scoring his first six points in the paint after missing jumper after jumper against the Nets.

He posted his seventh double-double of 14 points and 10 rebounds.

And the Celtics either had too much fun in Scottsdale or too much rest or too much of something else, because they missed 12 consecutive shots in the first quarter.

By the time any of them remotely woke up, the Suns had raced out to 19-point lead to end the first, and had led by as many as 22, but crumbled in crunch time to Irving and the Celtics’ comeback.

Crushed: Canaan’s turnaround fading bucket as the shot clock expired to put the Suns ahead by 14 with 9 minutes 43 seconds allowed the crowd to exhale for a second.

Canaan shrugged his shoulders almost in disbelief that it dropped.

After the game, he had that same look, but for a different reason.

Canaan shot 1 of 7 in the second half, missing all three of his 3-point attempts. So that was his lone bucket as he finished with seven points on a woeful 3-of-14 shooting.

“I try not to think about missing,” he said. “I believe in the next one going in. I know if I probably made one or two of them at the end, we probably wouldn’t be in that position (at the end of the regulation).”

Where’s Jackson?: Kokoskov said after Wednesday’s practice Josh Jackson, who played just seven minutes in Tuesday’s loss, could see more minutes against Boston.

“We’re playing a team as we know is a very good team that’s playing small ball,” he said. “Some games, he’s going to play much more. Maybe that’s tomorrow.”

Maybe not.

Jackson played just five minutes and didn’t see any action the second half or overtime. He made his only shot attempt, had three rebounds, two blocks, but also two turnovers.

Jackson only had two fouls at halftime. So, he wasn’t in foul trouble, but found himself planted on the bench for the remainder of the game.

After that same Wednesday practice, Jackson said he was just trying to get in where he fits in. He hasn’t been given much opportunity to do that in recent games.

Coaching move: Stevens watched Jason Tatum score zero points on 0-for-4 shooting and not be fully engaged defensively.

So Stevens started the second half with Marcus Smart, who defends and plays hard, not Tatum.

Most coaches wouldn’t sit a starter like that, but Stevens isn’t most coaches. That move helped set the tone defensively for the Celtics as that was the only way they were going to get back the game.

Boston limited Phoenix to 37.5 percent shooting in the second half after the Suns shot 51.2 percent in the first half in building a 20-point halftime lead.

Final line:  Phoenix shot 0 of 8 from 3 in the fourth quarter.

Ouch.

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