Marcus Morris took the inbound pass from Jayson Tatum with 6.1 seconds to go in regulation, with Devin Booker guarding Morris well above the top of the key. Then, Morris handed the ball to Kyrie Irving, and both Booker and Isaiah Canaan converged on Boston’s star guard as he took two dribbles to his left.

That left Morris wide open to get the ball back from Irving and drill a game-tying 3-pointer with 0.3 seconds remaining on the clock, sending the massive contingent of Celtics fans at Talking Stick Resort Arena into a frenzy. It completed Boston’s rally from 22 points down against Phoenix to push the contest into overtime, and the Celtics surged ahead to win 116-109. It spoiled a desperate Suns team’s opportunity to upend a bona fide NBA title contender two nights after getting drubbed by the lowly Brooklyn Nets on its home floor.

But why did Morris get free to launch that shot in the first place? Why didn’t Phoenix subscribe to basketball 101 — committing a non-shooting foul while up three points with less than seven seconds remaining on the game clock?

The Suns were supposed to foul a ballhandler at that spot on the court, coach Igor Kokoskov said. Or they weren’t, said Booker, Canaan and rookie big man Deandre Ayton. The discrepancy suggests either an unfathomable miscommunication from coach to team, or two teammates’ refusal to throw one of their own under the bus.

“You take the blame and the responsibility as a coach …” Kokoskov said “The coverage was to ‘black’ any ball screen, which is switching. And also, any catch that far out from the 3-point line, (the call) is to foul.

“(Your players) have to be careful when you commit that foul, because Kyrie was going downhill, very sneaky. Right now, the way (officials are) calling shooting fouls, you have to trust your guys. The call was (to) foul.”

Kokoskov and those three players all agreed the Suns were supposed to switch “everything” defensively, with Canaan acknowledging he “just lost sight of” Morris. But when specifically asked if they were supposed to foul on that possession, all three players said “no.”

“You get caught up in the game,” Canaan said, “and sometimes you just get over-aggressive and think that the best player on the team (Irving) is gonna take the shot. He passes it, and (Morris) made the shot.”

The critical miscue sent players back to a Suns bench — with a collective mood Ayton described as “shocked and quiet” — to attempt to regroup for overtime.

The Suns, whose effort and accountability levels were previously criticized in the locker room, had bolted out to a big lead, taking advantage of Boston’s 13 consecutive missed shots in the first quarter. Phoenix had held all Boston starters not named Irving to zero first-half points. Even as the Celtics made small charges following intermission, the Suns held a 14-point advantage with 3:45 left in the fourth quarter.

Booker was en route to season highs in points (38), assists (nine) and steals (three) as part of an electrifying duel with Irving, who finished with season-high 39 points, seven rebounds and six assists.

Yet fatigue caused the Suns’ late fizzle, their coach said. The two teams traded overtime buckets until the 2:20 mark. Then Booker misfired on four consecutive possessions, helping Boston ride its biggest lead to the end of the extra frame.

“You could tell they got tight,” Morris said of the Suns. “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.”

Added Kokoskov: “We stopped running our offense. We just (put) the ball into ‘Book’s’ hands, and he did a lot of good things for us when we were playing well. As a team, you want your best player to have the ball in his hands and make the decisions for you.”

Kokoskov and players tried to spin the heartbreaking loss positively, stressing the Suns will win a ton of games if they sustain their first-half performance for 48 minutes. But Phoenix “just fell apart down the stretch,” Booker said. And the Suns learned that even a “simple mistake,” as Ayton characterized it, in crunch time can yield deadly consequences against an opponent as talented and deep as Boston.

That defensive miscommunication left Morris wide open for the game-tying bucket, providing a final jolt of Celtics momentum needed to snag an improbable victory.

Did Morris expect the Suns to foul on that final possession of regulation?

“Maybe,” Morris said. “But they didn’t, and I made them pay.”

(Photo: Jennifer Stewart / USA Today Sports)