Alexander Romanov has long-term stability on Long Island now, having signed an eight-year, $50 million extension with the Islanders two days before free agency opened this summer.

The 25-year-old defenseman is not using that as a reason to let his attitude slip.

“Obviously so happy,” Romanov told The Post before skating in his first preseason game of the year, a 6-2 loss to the Devils Tuesday night. “But you got to prove that you’re responsible to play at this high level for the next eight years. We’re not only talking about this year, next year. All eight years.”

And what, exactly, does that mean on his part?

“Just play hard,” Romanov said. “Hardest hockey I ever played. Go all out.”

That is commensurate with the sense of responsibility Romanov has displayed ever since he arrived on Long Island from Montreal in a 2022 draft-day deal.

It looked then as though Romanov forming a long-term top pair with Noah Dobson was an eventuality, and for a while, things went according to that plan.

With both hitting restricted free agency over the same summer, though, it wasn’t just a cap squeeze to bring back both but — with the Islanders using the No. 1 overall selection on defenseman Matthew Schaefer — it was also no longer necessary to their long-term plans.

Dobson went to Montreal.

Romanov stayed, and if things work out as anticipated, he and Schaefer will be two-thirds of the Islanders’ left side through 2033.

“He’s a great player,” Romanov said of Schaefer. “The most important thing I noticed is that for his age, I don’t see a big difference between him and NHL players. It looks like he already has NHL experience.”

Kashawn Aitcheson was the only Islander to play in each of the first two preseason games, returning to the lineup after leaving Sunday’s match following a knee-on-knee hit from Flyers defenseman Noah Juulsen.

“Just caught my knee,” Aitcheson said of the hit. “Just hurt it a bit, but we’re good now.”

Marshall Warren traveled to New Jersey as an extra defenseman in case Aitcheson could not go.

Coach Patrick Roy said Anthony Duclair likely would play his first preseason game on Thursday at Madison Square Garden, but it will depend on how the winger feels after the Islanders rushed him back too soon from a torn groin last year.

“He needs to be 100 percent when he goes,” Roy said.

Asked about his dad, John MacLean, no longer being on the Islanders’ coaching staff, Kyle MacLean downplayed it as “not too big of a deal.”

“It’s good for both of us, establishing myself, and maybe the separation will be good,” Kyle MacLean said. “It’s a business. We get it. He gets it too.”

Forward Daylan Kuefler left the game in the first period with an upper-body injury. Roy did not have an update on his status after the game.