What a debut.

Reggie Bullock truly became a Knick with 3:52 left in the first quarter when he checked into his first game for the franchise that signed him in July. With plenty of family members at the Garden, Bullock received cheers — and kept them lasting.

Bullock drained his first shot from the top of the key, then he buried a 3-pointer. After battling back from spinal fusion surgery in July and the murder of his sister in Baltimore in the fall, Bullock showed no rust. He shot 4 of 9 from the field — 3 of 5 from the 3-point stripe — to finish with 11 points in 15 minutes.

The best part?

“My son knowing I was playing tonight, he stayed up to watch the game to see his Dad,’’ Bullock said. “My family, we’ve really been through a lot. Go out there with family there behind my back, just trying to do something for the rest of the family and take the negative things away and do something positive.”

Interim Knicks coach Mike Miller thought it would take a few games before Bullock looked like himself, a career 38.7 percent 3-point shooter.

“He gave us a great boost,’’ Miller said. “He came in and made a couple of shots but made a defensive play right away getting through a screen [to stay with] CJ McCollum. To be fair, it was his first game back and we thought it would take a couple of weeks to really get his legs under him, but I thought he was really good.’’

This is the first time the Knicks have had all of their shooting guards available — all five. Miller has a dilemma. RJ Barrett played 25 minutes, Damyean Dotson got in for 13:16 while neither Wayne Ellington nor Allonzo Trier got off the bench.

“Open shots, that’s my gift,’’ Bullock said. “That’s what got myself into this league. Dealing with the neck injury and still being able to shoot fluently, I’m going to keep building. One game, 47 more to go.’’

Dotson said beforehand the logjam would be tough on Miller.

“It’s the dilemma coming into the season — what was going to happen, how it will work out,’’ Dotson said. “We’ll see. It’s the first game.’’

Dennis Smith Jr. missed his third straight game with a strained oblique. The Knicks are 5-1 under Miller in games Smith has missed. … The Knicks’ bench outscored Portland’s 64-13. … Julius Randle kept up his hot hand with 22 points, including 3 of 6 from 3-point land.

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