What a 285-pound letdown.

Cancel Zion Williamson’s first NBA game at the Garden on Friday. The Pelicans announced Thursday afternoon that Williamson didn’t make the trip to Manhattan because of right knee soreness with the ominous detail he was getting further testing and evaluation.

Williamson’s durability figures to be the only thing standing in the way of greatness because of his heavy build at 6-foot-7 and 285 pounds.

Maybe there’s something about Williamson and an aversion to the Knicks. Williamson had looked unstoppable in preseason after that abbreviated summer league showing in July against the Knicks in which he lasted just nine minutes before the Pelicans shut him down with a sore knee and back.

He looked overweight, but reportedly came to training camp slimmer.

On Friday morning, Alvin Gentry sounded like a coach who will be without his No. 1 pick for a while. Gentry said he can’t commit to a timetable yet for Williamson.

Gentry said he talked to Williamson in the morning and reported the youngster’s spirits were fine.

“He’s not dead, guys,’’ Gentry said after the shootaround at the Garden. “He’s a typical 19-year-old kid, he’s upbeat, he has fun and he’s living his dream.”

Knicks rookie RJ Barrett had expected to reach out to his former Duke roommate on the eve of Friday’s preseason Garden showdown to hang out. That won’t happen.

Barrett was already giving a tip to his teammates on how to defend his best friend with the preseason finale between the Knicks (1-2) and 4-0 Pelicans on tap.

“You’ve got to try to make his finishes tough at the rim,’’ Barrett said after Thursday’s practice in Tarrytown. “That’s about all the advice I could give.’

“He’s a monster,’’ Knicks coach David Fizdale said before learning Williamson won’t play. “It’s exactly what everybody thought he was going to be. He’s a beast. He’s just going to continue to get better and better. We understand that kid is a heck of a player.”

The No. 1 pick in the draft had been devastating since his Las Vegas hiccup in July. In the last three preseason games, he shot 29-of-36, showing terrific efficiency.

The 19-year lefty notched 22 points on 11 shots against San Antonio. He put up 26 points on 12 shots against Utah. He’s been a lefty dunkmaster and mid-range titan.

Barrett, who was 4 of 14 against Atlanta on Wednesday, missed double the shots in that one contest that Williamson has in three games. The Knicks’ No. 3 pick is not surprised at all by Williamson’s success.

“He’s too big, too strong,’’ Barrett said before the injury news. “He’s going to continue to do that hopefully over his whole career.’’

If he stays healthy.

The Garden was expecting another sellout to watch the phenom. TNT is scheduled to televise the game. In the last three games, Williamson averaged 25.6 points. 8.3 rebounds 6.7 assists.

TNT’s Stan Van Gundy said the best part of Zion is in interviews, he’s “always talking about his teammates.’’

“He looks pretty good so far,’’ said ex-Pelican Julius Randle, who watched Williamson’s summer league cameo and was set to start out defending him on Friday. “Everybody had questions. I saw him close up in summer league and felt he’s going to be fine because he plays really hard. You got guys like Lonzo [Ball] and Jrue [Holiday] who will make his life that much easier. He’ll definitely benefit from that as well.”

Now, assuming Williamson is healthy, he will play his first game at the Garden with the Pelicans on Jan. 10.

Randle has no ill will for the Pelicans, where he starred last season but was not re-signed. He resuscitated his career with New Orleans, but the Pelicans wanted no part of the three-year, $63 million the Knicks lavished on the one-time Lakers lottery pick.

Asked if the game was special, Randle said, “No. It ain’t extra. I love those guys over there. It was extra in LA [with the Lakers] for me but it wasn’t extra in New Orleans. That situation ended great. I had a great time, a great year over there.”

Barrett has played a decent preseason himself, though his perimeter shot is still a work in progress. Barrett is averaging 14.7 points, 3.3 assists and 7.0 rebounds. He struggled with his shot Wednesday, but got to the basket with fierceness and missed a few easy ones.

“I was great with how it went,’’ said Barrett, who also had seven rebounds and six assists. “Easy layups, easy stuff, wide-open stuff, that will go down, that happens to every player. I was happy with the shots I got.”

Now Barrett has center stage.