Never before has a star left for a neighbor

IT is essentially impossible and would be thoroughly unprecedented.

Just so you understand that. There doesn’t seem to be one good reason, even up to 10 million good reasons, why he would want to do it. Just so you understand that, too. But what if ? What if — after nine years of pledging his heart, soul and loyalty to the Islanders only to be repaid with unvarnished organizational incompetence — John Tavares decides that, yes, indeed, he wants to see life from the other side?

What if No. 91 takes the route never traveled and signs as a free agent on July 1 with the Rangers?

Never in the history of our town — which has had at least two major league baseball teams for 112 years, at least two big league hockey teams for 63 years, two pro football teams for 84 seasons and two pro basketball teams for 51 years — never, ever has one franchise’s signature player in his prime gone directly from one to another. Just so you know. The Dodgers actually tried to trade Jackie Robinson to the Giants at the end, but No. 42 was literally at the end and retired. Doc Gooden and Darryl Strawberry went from Queens to The Bronx, but took complex and circuitous routes to get there and were only prime in pinstripes by measure of their celebrity. (Though Straw could still hit, couldn’t he?) David Cone had previously jumped on the shuttle, but with stops between in Kansas City and Toronto.

Bernard King went from Piscataway to the Garden, but spent time in the interim with Utah and Golden State and wasn’t quite Bernard King in Jersey, was he? Bobby Holik and Scott Gomez traded in Lou Lamoriello’s prison garb for the bright lights across the river and were hardly ever heard from again. Dave Jennings took his ball from the Giants to the Jets, but he was a punter. Don Maynard wasn’t even an afterthought in Blue before becoming a legend in Green.

Leo Durocher went directly from the home dugout in Ebbets Field to the one in the Polo Grounds in 1948 in the most shocking case of intramural activity, but for all his prominence, he was, after all, a manager. So, no. Signing with the Rangers is the last thing anyone would expect from Tavares, who might have played his final home game as an Islander in Thursday’s 2-1 victory over the Blueshirts at Barclays Center.

And that includes the Rangers. There is no plan in place to seduce the Islanders’ splendid 27-year-old captain into becoming the Blueshirts’ 28-year-old captain (birthday during training camp) anymore than there was ever a plan to entice Zach Parise into switching sides when the then-Devils captain became available in 2012.

But Jim Dolan, Glen Sather and Jeff Gorton sure would listen if Tavares invited them to the party, sure would be willing to adjust their rebuild to accommodate this stud, the likes of whom the Rangers haven’t had since Jaromir Jagr in 2005-06. Sure would be willing to send a different type of letter to the season ticket-holders. Of course they would. The Maple Leafs, whose rebuild under Brendan Shanahan had only begun, were not especially anxious to accelerate their program and calendar by getting in on the Steven Stamkos sweepstakes when it appeared as if the Tampa Bay captain was going to explore the open market in 2016. But Toronto did woo that No. 91 during the short window before he declared his undying loyalty to the Stanley Cup contender in the no-tax state with whom he’d played his entire career. Of course they did. No one says no to a talent like Stamkos. No one would say no to a talent like Tavares.

But what would be next, other than the ultimate humiliation of and for the Islanders, if the unthinkable occurred?

The Blueshirts would have to do whatever they could to add a first-pair right defenseman — either through a trade that would almost necessarily include either Mika Zibanejad or Kevin Hayes, or through signing free agent John Carlson. It would make the Rangers’ recruitment of Ilya Kovalchuk, who wants to come back to the league with a team that has a shot to win, a whole lot smoother. And you could count on a trade of at least one of the prospects and one of the late first-rounders acquired last month, if not both.

The Rangers would be reloading, not retooling, not rebuilding. They’d mount up for another charge in front of Henrik Lundqvist, this time with a bona fide superstar to lead the way. But this, of course, will not happen. Something like this has never happened. Just so you understand.