New York Islanders' Frans Nielsen (51) and Kyle Okposo, center,...

New York Islanders' Frans Nielsen (51) and Kyle Okposo, center, celebrate with John Tavares, right, after Tavares scored during the second period of an NHL hockey game against the Buffalo Sabres, Thursday Dec. 31, 2015, in Buffalo, N.Y. Credit: AP / Gary Wiepert

Islanders captain John Tavares knows how much his team has changed this summer. Although longtime teammates and friends Kyle Okposo, Frans Nielsen and Matt Martin departed in free agency July 1 and Tavares knew long before then that there was a chance all three could leave, it’s still hard for him to believe.

“It hasn’t even really sunk in, maybe until you don’t see them at training camp or during the season, and that hasn’t happened yet,” Tavares told Newsday last week. “I don’t think people realize how much I lean on Kyle and Frans, as well as guys like Travis [Hamonic], Clutter [Cal Clutterbuck], Johnny Boychuk. Having those different perspectives is a huge help. And personally for me, you go through a lot of ups and downs and you’re trying to manage a lot of things. When you have people you can rely on, especially with those friendships I have with Kyle and Frans are deep.”

The challenge now, with a little more than a month until training camp begins, is how to handle the loss of three such deeply ingrained players in the fabric of this organization — Okposo and Nielsen were Tavares’ alternate captains the past three seasons — while integrating three veterans in Andrew Ladd, Jason Chimera and PA Parenteau.

Oh, and there’s another wrinkle: When players arrive for physicals on Sept. 21, the captain, the coach and their two experienced goaltenders won’t be around.

Tavares, coach Jack Capuano and goaltenders Jaroslav Halak and Thomas Greiss will be at the World Cup in Toronto, an event that certainly will draw positive attention when it starts on Sept. 17 but will make 30 NHL general managers nervous because of the potential for injury and how disruptive it might be for NHL training camps.

“Our coaching staff understands me well. There’s even times we agree to disagree on things, but we all have the same goal when it comes to the hockey club,” said Capuano, who will be an assistant to John Tortorella for Team USA. “I’m sure the start of camp will be very efficient, very structured and we’ll be able to get off to a quick start with what the coaches and I have discussed. There’s a ton of respect in our room from the players to the assistants, and I don’t have much worry there.”

Tavares isn’t sulking about the departure of his friends — Okposo to the Sabres, Nielsen to the Red Wings and Martin to the Maple Leafs — and has reiterated his desire to stay with the team beyond the end of his current contract in 2017-18. He was a big part of the team’s pitch to college free agent Jimmy Vesey, who signed with the Rangers on Friday.

And Tavares has no concerns that the room will be fractured without his close confidants.

“It’ll feel different, but we’ve all been through changes and I’ve come to grips with what’s happened,’’ he said. “I’m happy to see those guys get rewarded. They’re looking forward to their new opportunities and it’ll be different not seeing them every day, but in terms of involving new players and relying on them heavily, that’s happened before. When Johnny and Nick [Leddy] came to the team, they fit right in from day one and had an enormous impact on our team . . .

“I know Ladder a bit — he’s got a great family, he’s a leader and the two Cups he’s won, he’s been through a lot in his career. He’s someone to definitely really count on and will make a great impact in our room. Someone who’s been through it and succeeded will be great for our young guys. He’ll be someone I lean on in a lot of areas.”

Capuano said the staff hasn’t chosen new alternate captains, though Clutterbuck and Boychuk have worn the “A’’ before and Hamonic and Ladd likely will be in the mix.

“It’s not just about who has a letter,” Capuano said. “Our leadership group, they’ve matured over the years. Johnny’s matured into one of the best captains in the league, in my opinion. We have a lot of guys who can be part of that, not just veteran guys but hopefully some of our younger guys as well.”

It will be a different room without the soft-spoken but direct Nielsen, the cerebral Okposo and Martin, the fun-loving kid brother who liked to keep everyone loose. Change is hard. It also can be good, especially for a team that has played well the past two seasons but wants to take a real leap to the upper echelon of the Eastern Conference.

“Guys I think understand the culture we’ve developed, where we’ve established ourselves,” Capuano said. “Having some new blood come in, it does goose you a little bit, it can bring some excitement to the room. We saw three quality people go — not just good players, but people who represented the best of this organization. And we brought in three quality guys who can do the same.”

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