PHOENIX — Kristaps Porzingis’ road to superstardom has hit a speed bump.

It’s either a more severe attack of the rookie wall — which is understandable — or not taking well mentally to the Knicks’ sudden coaching change the day after the Super Bowl.

That was the last time Porzingis posted a double-double — his 18th of the season — in Derek Fisher’s final game as coach. He has yet to hit that marker in 11 games under interim coach Kurt Rambis.

On Wednesday, Porzingis was more active early and got to the free- throw line lots but didn’t shoot well and at times got outmuscled by ex-Knick Tyson Chandler, who clearly was trying to send a physical message to the rookie. Porzingis finished with 13 points and five rebounds, shooting 2-of-10 from the field and making 8-of-9 free throws in the Knicks’ 128-97 blowout win over the Suns.

“Everybody’s trying to be physical with me,’’ Porzingis said after the game. “Whenever I’m in the low post or around the basket, [Chandler] tried to do the same thing. I missed some shots I normally make. It was a fun battle. He said some things a little bit.’’

Is Rambis worried? No matter how constructively critical he has been, Rambis still sees an eventual superstar in Porzingis — just not right now, when he needed him to form a dynamic duo with Carmelo Anthony to infuse life into a playoff push that is now dead.

“I’m not concerned at all,’’ Rambis had said after Tuesday’s 110-94 Denver debacle. “He’s still 20 years old. He’s got a lot to learn. My mindset is not going to change about him. He’s going to be great. There’s not a doubt in my mind. But if you thought it was going to happen this season and be perfect and not make mistakes, do everything right and solve all the mysteries of basketball this year, it wasn’t going to happen.

“He’s got a lot to learn. He’s going to get better every year. And he’s going to go through this NBA process. He’s going to spend this summer getting better, bigger, stronger each year. He’s going to be a better, better player.’’

Since the All-Star break, Porzingis has had just one vintage game — in a 108-105 defeat in Indiana on Feb. 24 when he notched 22 points. Had Anthony not experienced a horrible shooting night (5-of-20), Porzingis would have been the star of a marquee win.

On Tuesday, Porzingis took another step back. He returned from his bruised shin and played poorly, notching six points, hitting 3-of- 11 shots, committing three turnovers, securing five rebounds with two blocks.

“I should let the game to come to me, and it starts with rebounds, defense and shot blocks and the game will come to me,’’ Porzingis said.

The rookie was hard on himself when he was blazing hot the first two months of the season. Now he is trying to downplay his recent decline. Last season in the Spanish League, he played 50 games. He played his 64th game Wednesday.

Fisher always noted the Knicks were a lot better when Porzingis was on the floor, but it’s interesting to note in their best game since the All-Star break, a rout of the Pistons on March 5, Porzingis didn’t play.

“It happens,’’ Porzingis said. “Bad games happen. Two, three games later, I have a great game. It’s the ups and downs, especially my rookie season. It’s all learning experiences. Sometimes games you don’t feel as well, some games you feel better. I’m trying to find consistency to play at the same level. I’m not there, yet. But I’m learning.” “He’s going to be challenged to defend bigger guys inside and contest 3-point shooters,’’ Rambis added, referring to Porzingis’ dual position as a power forward/center.

Rambis said for the first time that Porzingis was too shot-happy. It appeared the opposite early in the season.

“Sometimes he gets a little too eager looking for his shot,’’ Rambis said. “He’s got to be more patient, allow things to come to him. I understand his frustration. We got to do a better job getting him involved more, but he still can’t go off on his own, looking for his own shot.

Rambis said he’d rather Porzingis answer the question of whether he’s fatigued, but the coach indicated it would be normal if he were. Besides being less active defensively, Porzingis’ shooting legs aren’t there.

He’s down to 42.4 percent from the field — 33.3 percent from beyond the 3-point arc. His scoring average has dipped to 13.9 points while averaging 7.3 rebounds and 1.9 blocks.

“You have to ask him that,’’ Rambis said. “I’m sure the grind of the NBA season wears on all guys. It’s tough to manufacture the energy it takes to play well in every single ballgame. He’s one of those guys, he’s highly touted, and everybody’s going to go after him. They’re going to try to do their best against him. He’s seeing everyone’s best. “He’s going to be all over the court in terms of defense. He’s going to get better in all that, but if you ask me if he feels tired, he’s played more games than he ever played in a basketball season.’’