Jayson Tatum had no shot.

Nothing.

He drove into traffic with a wobbly handle and zero angle. The Knicks swarmed him with three players and three seconds remaining in the left corner. He collapsed like a dying star.

If you want to know why the Celtics are staring at a 0-2 series deficit unforeseen by the entire basketball-watching world, start there.

Start with the dying star.

Through two games, the Celtics have been out-scored with Tatum on the floor. He scored 13 points on 19 shots spread over almost 42 minutes of action Wednesday in Game 2. He sunk the Celtics’ chances at the end of Game 1, chucking low-percentage 3-pointers in the final minutes because he wanted the Kobe-esque step-back instead of a hard-charging drive that could have resulted in a layup or foul shots.

Tatum is the problem.

Not bad shooting luck, nor an injured supporting cast, though both have obviously hamstrung the Celtics’ offense for long stretches. The reigning NBA champions are two games from elimination against a team with fundamental defensive problems that they swept in the regular season because their superstar is flailing.

Tatum’s touch appears to be gone. He’s shooting 28.5% from the floor and 25% from 3. The Knicks’ cycling of defensive schemes — from switching to drop coverage to meeting him at the level of the screen — is unsettling him. A top-5 player in the world looks lost.

Even his go-ahead basket with 18.5 seconds left was a gift. Joe Mazzulla designed a play for for Tatum to take the inbounds pass, then an Al Horford screen at mid-court to allow him a runway to the rim against lumbering Knicks center Mitchell Robinson. Once Tatum got past Robinson, it was over. Any decent ball-handler in the NBA could have flushed that dunk to go up 90-89.

But trailing by one and left to his own devices, left to hunt Robinson again on a switch during the last possession, Tatum failed.

Just like the end of regulation in Game 1.

Meanwhile, his star counterpart soars. Jalen Brunson is hunting and killing, even Jrue Holiday.

Unable to shake Holiday with a screen on the Knicks’ last possession, he danced away from Holiday on his own and then jumped in front of him near the foul line. This put Holiday, an All-Defense player in an impossible position. Holiday did the smart thing because it was the only thing, and fouled Brunson before he could get a clean look.

There is no shame there, even if Brunson, an 82% free throw shooter, canned both shots to take the lead. But there is enough shame for everyone involved when blowing a 20-point lead for a second straight playoff game; a first in NBA history.

Because collapses start with cracks. Cracks foretell trouble, sometimes doom.

After the Celtics led by 20 late in the third quarter, then 16 with 8:40 remaining, cracks surfaced.

The Knicks trimmed the Celtics’ lead to nine, then seven, then four in the final minutes. Missed rotations and poor offensive pace followed. A nervous murmur in the Garden yielded to an anxious roar, as if the crowd could scream confidence back into its team.

No dice.

Boston’s Jayson Tatum, left, and OG Anunoby of the New York Knicks go after a loose ball during the second half of Game 2 of the Eastern Conference semifinals at the Garden. (Photo By Matt Stone/Boston Herald)

The Celtics missed a wave of 3s. Brown missed an above-the-break triple after Tatum whiffed on a wide-open look in the right corner and Derrick White couldn’t connect on a catch-and-shoot 3 from the left wing, all right in a row.

Bad timing, back luck.

But once the Knicks closed within one, Tatum attacked OG Anunoby, New York’s best defender, in a 1-on-1 matchup. He settled for a fall-away along the right baseline.

Bad decision. Miss.

Next, Brunson buried a go-ahead bucket at the other end, and Brown responded with an off-balance mid-range jumper, hoping, praying for a whistle.

Bad execution. Miss.

On and on it went, a slow, painful basketball death authored primarily by the Celtics’ biggest stars. Brown at least is hurt. Tatum has no excuse. Not even for his play in the first half.

After the Celtics predictably rolled to a double-digit lead in the first quarter, driven by their desperation, play slowed and the lead shrank. Tatum continued shooting like he was missing a finger, and walked into halftime with just two points. The Knicks hung around, clinging to a confidence the Celtics now can never extinguish even if they rip off three straight blowouts.

The Knicks know they can win this series. Hell, a sweep is on the table, as unforeseen and impossible as that sounded 48 hours ago.

But this is the reality the Celtics face now, no thanks to their best player.

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Not to mention, other problems abound heading into Game 3.

Kristaps Porzingis went minus-9 in his 14 minutes of action Wednesday. He took zero shots and collected one rebound in the first half. He played like a ghost; a ghost who declined to post up backup Knicks guard Cam Payne on his final possession before halftime, despite a 10-inch height advantage.

Mazzulla only re-entered Porzingis after the Celtics had mounted what felt like a safe, 18-point lead in the third quarter. Porzingis played several minutes with Horford, but the Knicks started to win those double-big battles ultimately forcing him back off the floor.

Porzingis was listed as probable before tip-off. It seems probable now he won’t return to form any time soon, and maybe for the entire series, thanks to his ongoing, mystery illness.

That will leave Tatum left to pick up the slack and cover the cracks.

The star in the No. 0 jersey, who’s played like a zero all series.