Jayson Tatum had no plan.

No shot.

Nothing.

He slowly drove into traffic, the Knicks swarmed him with three players and three seconds left. He collapsed like a dying star.

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

Tatum has been a no-show this series. A complete disaster.

Through two games, the Celtics have been out-scored with him on the floor. He scored 13 points on 19 shots spread over nearly 42 minutes in Game 2. He sunk the Celtics’ chances at the end of Game 1, chucking low-percentage 3-pointers in the final two-plus 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.

It’s Tatum.

Not bad shooting luck. Not an injured supporting cast. The reigning NBA champions are two games from elimination against a team with permanent defensive problems that they swept in the regular season because their superstar is flailing.

Even Tatum’s go-ahead basket with 18.5 seconds left was a coaching gift. The play called for Tatum to take the inbounds pass, then an Al Horford screen at mid-court that allowed Tatum a runway to the rim against lumbering Knicks center Mitchell Robinson. Once Tatum got past Robinson, any decent ball-handler in the NBA could have finished that dunk.

But left to his own devices, left to hunt the Robinson switch again in the following possession, Tatum failed.

Same as Game 1.

Meanwhile, his star counterpart is soaring. Jalen Brunson is hunting mismatches to kill, 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. 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 go ahead. But there is enough shame for everyone involved when blowing a 20-point lead for a second straight playoff game.

Because collapses start with cracks. Cracks foretelling trouble ahead. Signs that something should be down

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

The Knicks trimmed the Celtics’ lead to nine, then seven, then four in the final minutes. A nervous murmur in the Garden yielded to an anxious roar trying to scream confidence back into their team.

No dice.

The Celtics missed a wave of 3s. Brown missed a head-on 3-pointer, 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.

Once the Knicks closed within one, Tatum attacked OG Anunoby, New York’s best defender, for a fall-away jumper.

Bad decision. Miss.

So Brunson buried a go-ahead bucket at the other end, and Brown attacked him with a mid-range shot looking for contact.

Bad decision. Miss.

On and on it went, a slow, painful basketball death authored 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 desperation, play slowed and the lead shrank. Tatum continued shooting like he was missing a finger, and walked into halftime with two points. The Knicks hung around, clinging to a confidence the Celtics can’t rob them of.

They 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, dragged down by their best player.

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

Kristaps Porzingis went minus-9 in his 14 minutes of game action. 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 boasting a 10-inch height advantage on the block.

Mazzulla only re-entered Porzingis once the Celtics had mounted an 18-point lead in the third quarter. He played frequently with Horford, but the Knicks started to win those double-big battles forcing Porzingis back off the flor.

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 mystery illness.

That will leave Tatum left to pick up the slack.

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