Knicks rookie point guard Frank Ntilikina injured his left anckle...

Knicks rookie point guard Frank Ntilikina injured his left anckle during practice at the team's training facility in Westchester on Oct. 20, 2017. Credit: Newsday / Barbara Barker

Frank Ntilikina’s NBA career isn’t starting off the way he hoped.

The rookie point guard did not play in the Knicks’ home opener Saturday night because of a left ankle sprain he suffered after practice Friday. Ntilikina played only one preseason game because of a knee issue. He also had a rough debut Thursday in Oklahoma City, air-balling his only two shot attempts.

“As a player, you always want to go on the court and play basketball,” Ntilikina said. “It’s a little bit frustrating. I have to be strong mentally and I have to keep going, keep working and keep learning and do the best I can do off the court.’’

Ntilikina, whom the Knicks took with the No. 8 overall pick, rolled his ankle while boxing out Willy Hernangomez during a four-on-four scrimmage Friday. He had to be helped to the locker room.

Ntilikina said he’s “a little bit upset” that he can’t play but relieved that it’s not that serious. He said it didn’t swell up much overnight. He ran during shootaround but had trouble going side-to-side, and hopes to play Tuesday at Boston.

“It’s nothing serious,” he said. “On the play, I felt it a lot. Ten minutes after, I was OK . . . It’s not as bad as it can be.

“I was a little bit scared about it. After the night, I was confident it wasn’t very serious.”

One good thing did happen for Ntilikina: The huge Nike ad featuring his likeness on the side of the skyscraper outside of Madison Square Garden was completed.

“I saw it,” he said. “It’s crazy. I love it. It’s beautiful.’’

Unhappy Hernangomez

Hernangomez admitted he was “mad” when Jeff Hornacek told him he wasn’t in the rotation. The second-year center played only garbage minutes at the end of Thursday’s opener as Enes Kanter and Kyle O’Quinn got the minutes over him. “I was mad,” Hernangomez said. “I think we are a rebuilding team. I have to keep working and wait for my moment. I know I’m ready. I think I showed last year I can play here.”

It was surprising because Knicks officials touted Hernangomez as one of the cornerstone players, but Hornacek said management told him, “Play the guys that have earned it in practice.” He wants Hernangomez to be more impactful on defense.

“Willy’s done well, just something the other guys have done a little bit more,” Hornacek said. “So the focus is on defense, and that’s one thing that we want him to get better at as a young player.

“It’s a tough spot. But that’s when guys step up. I think Willy will too.”

Fast breaks

Michael Beasley, who played only 36 seconds before hurting his ankle in the opener, returned to the lineup . . . With Ntilikina out, the Knicks activated veteran point guard Jarrett Jack.