It started 18-1 and it finished 114-73, an endless afternoon of basketball malpractice by the Knicks, an egregious performance in which nobody was spared, least of all the 19,812 people inside Madison Square Garden forced to witness the bloody carnage the Celtics inflicted across 48 joyless minutes.

“I didn’t see this coming,” Mike Woodson said, and one more time the coach of the Knicks discovers there is no more dangerous a proposition with his team than taking a deep breath, believing the worst is past, and embracing prosperity.

He says that’s on him: “I can’t have guys step on the floor like that.”

His players say it’s on them: “That’s on us,” J.R. Smith insisted. “That’s our job as professionals.”

It’s a perfectly pointless game of accountability pingpong that doesn’t do a thing to address the fundamental problems of this team, which spent three days delighting in the “new” system it believed it’d mastered on the fly, one of defense and pace and hit-the-open man, one the Knicks insisted to a man they’d embraced.

That dissolved like antacid tablets somewhere between Friday night and Sunday morning. And while some wanted to blame the Celtics’ torrid shooting and others chose to point to their own frigid offensive game and still others were a little too willing to blame a lack of passion and effort, the truth was plainer than that:

On the Knicks’ very first possession — the crowd still anxious, still willing to give the benefit of the doubt based on a couple of thrashings of the Nets and Magic, the first honest-to-god “winning streak” of the season — the Knicks walked the ball upcourt (old behavior), handed the ball to Carmelo Anthony (very old behavior) and stood around until Melo launched — and missed — a 20-footer. Infuriating behavior.

And damned if you didn’t hear a murmur at that precise moment.

That murmur? That was the Knicks regressing — sprinting, really — back to the mean.

“I don’t understand it,” Anthony said, after turning in what could well have been the worst performance of his Knicks tenure if not by his modest numbers (19 points, five rebounds) than by the fact his plus-minus was a staggering minus-40, the worst of his career.

Actually, it isn’t that hard to understand and, in truth, there was an easy decoder conveniently located inside the Garden. The Celtics are supposed to be in transition, supposed to be marking time until their influx of draft picks yields a mother lode of talent, but the team they presently have is now 10-12, first place in the Atlantic, and more to the point they clearly buy what their coach, Brad Stevens, is selling.

It may not last across 82 games in a league where talent generally carries the day, but for now it’s a wonderful contrast to whatever it is the Knicks are doing day-to-day.

“We’ve been together from the start, learning about each other, players and coaching staff,” said Jordan Crawford, who obliterated the Knicks for 23 points, and a player talking about a coach in that context, after a 41-pont win … that carries a little weight and a lot of credibility.

The Knicks?

“I know Coach,” Anthony said. “I know what kind of guy he is. We’re not giving ourselves a chance when we step on the court.”