SAN JOSE, Calif. — The head vs. heart calculus which has tugged at the Islanders’ decision-making, or lack thereof, over the past month finally hit an inflection point late Thursday evening.

The Islanders said goodbye to Brock Nelson, trading one of the deadline’s biggest dominoes to Colorado along with prospect William Dufour for Cal Ritchie, Oliver Kylington, a first-round pick in either 2026 or 2027 and a conditional third-round pick in 2028.

In short order, the Islanders then flipped Kylington to the Ducks for future considerations

The first-round pick is top-10 protected in 2026, and would also become a 2027 pick if Colorado’s first-round pick this year — which they traded to Philadelphia with top-10 protection a year ago — does not go to the Flyers, who would then get the 2026 pick.

The third-round pick will convey if the Avalanche win the Stanley Cup this season and Nelson plays in at least 50 percent of their playoff games.

Brock Nelson is pictured during the Islanders’ game against the Predators on March 1. Heather Khalifa for the NY Post

The Islanders retained 50 percent of the money left on Nelson’s contract, equivalent to a $3 million cap hit, as part of the deal.

This was a moment, and this is now a trade deadline, four years in the making.

The Islanders resisted fundamental change to the core ever since a COVID-19 outbreak derailed their 2021-22 season, drifting into mediocrity as a result.

There is no more fundamental change than moving Nelson, a career Islander and alternate captain whose number should one day hang in the UBS Arena rafters.

Brock Nelson shoots the puck during the Islanders’ game on Jan. 2. Robert Sabo for NY Post

This was the move the Islanders did not make in 2018, when they hung onto John Tavares in the doomed hope that he would re-sign that summer.

This was the prudent move, and the one that will tug on the heartstrings for a fan base that watched Nelson grow up on Long Island.

And in a seller’s market, the Islanders’ return looks like a haul, with Ritchie — a 2023 first-round pick considered to be an elite prospect — as the centerpiece.

A native of Oakville, Ontario, Ritchie made his NHL debut in October, playing seven games with the Avalanche before going back to his OHL club in Oshawa.

The 20-year-old has 67 points (14 goals, 53 assists) this season and played with Team Canada at the World Junior Championships.

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Cal Ritchie is pictured during Team Canada’s game against Czechia on Dec. 23, 2024. AP

Getting a first-round pick as well amounts to the cherry on top of this deal that will begin to set the tone for the Islanders’ future.

All indications were that the Islanders were trying their hardest to sign Nelson to an extension as late as Thursday.

But nothing got done.

As much as Lou Lamoriello has stated and restated his belief in this roster, the general manager could not justify keeping Nelson as a self-rental only to watch him walk out the door this summer for nothing.

All focus now turns to Kyle Palmieri, the other top-six Islanders forward whose contract expires at the end of this season.

Islanders general manager Lou Lamoriello, pictured in October 2024, made his first Islanders move ahead of the trade deadline. Corey Sipkin for the NY Post

While the return for Palmieri wouldn’t be as much as the Isles got back for Nelson, the lack of extension there as Thursday night became Friday morning indicated that a move could be coming there as well.

Given the way the market has operated so far, the bare minimum the Islanders should expect back for Palmieri would be a second and fourth-round pick.

It is not, however, out of bounds to think a first-rounder could be in play if the Isles do indeed decide to move Palmieri.

Much as Lamoriello will be loath to set the Islanders back in the playoff chase this season — the team was still four points back of a spot after Thursday’s games, with Detroit, Columbus, Philadelphia, Boston and Montreal all losing — the long-awaited pivot to the future has finally arrived.

This was reality taking a hard-earned victory lap.

It will take another Friday if Palmieri is dealt too.