But, "we're together forever," he tells 106.7 The Fan, noting the eternal bond he'll carry on with the teammates with whom he hoisted the Cup last summer.
Niskanen, who signed a seven-year, $40.25 million deal to join the Capitals in July 2014, knew there was a chance he'd be traded given the reality of the Caps' salary cap situation and him being on a long-term deal.
"I'll go with I knew it was possible," he said. "I knew the cap situation in Washington, and I can do math, and I know what the makeup of our team is. I knew it was possible. I guess I was kind of hoping to get another year there before this happened. It's not unlikely, or uncommon, I should say, for guys on long-term deals to kind of get moved out towards the end of them. That's part of the gig."
"I wasn't totally shocked," he said. "But like I said, I was hoping for at least another year and have another crack at it with the Capitals. But knew it was possible. Brian MacLellan and my agent called me last evening and kind of told me the news. A little surprised, but that's part of the job and I knew it was a possibility.
"I guess thankful that it happened in the summer where it makes the move a little bit easier, rather than mid-season or during training camp or something. Yeah. Things happen. It's part of the job. I'll be a pro. And what were you saying to me? I guess I'm going to Philly. Here we go."
"I have nothing but good things to say about the organization, the city, fans," Niskanen said of his time in Washington. "The way I was treated there was just fantastic. I have no regrets. We had a great team for the five years that I was there – two Presidents' trophies, and winning the Cup in '18 obviously takes the cake. I just, great memories there. I was part of something really special.
"And like I said, thanks to the organization for giving me a chance and bringing me there. The fans and city treated me well. Like you said, saying goodbye to your close buddies and your teammates, that's hard. I mean I have a couple texts that I've gotten so far. I just missed a phone call."
Niskanen is looking forward to the inevitable reunion of last year's Stanley Cup team.
"It'll be really cool I guess nine years from now when we have a reunion and a couple of us are fat," he joked. "It'll be funny. It'll be great to see those guys again. Gotta play against them for a few more years here now, unfortunately, but a lot of good memories and kind of a tough day."
When one of the hosts joked that Alex Ovechkin will likely get fat once his playing career is over, Niskanen laughed, agreeing that that's probably a safe bet.
"I would guess so. I always say I'm gonna be 250," he said. "I'm gonna put on 50 and Ovi might put on even more."
Niskanen has been traded before. In Feb. 2011, he was shipped from Dallas to Pittsburgh, where he played three-plus seasons for the perennial Cup-contending Penguins. And while he was grateful for his time up north and for the Pens for giving him a chance to turn his career around, leaving Washington brings a distinctly different feeling.
"Coming to Washington, a little bit different type of group," he said. "In four years we ended up just climbing and climbing, and we ended up winning the whole thing. I think 2018 and winning the Stanley Cup together, that's something on a whole nother level.
"It'll be weird, really weird playing against the old teammates and my friends and stuff. There's one side of me that's got to be a pro, and go after 'em and try to shut 'em down and all that, but there's gonna be some weird feelings that first night. Part of the game, though. Part of being in the league. But like I said, no regrets and a lot of great memories there, and topped it off with 2018 Stanley Cup champions. Unfortunately, we're moving on."
"I'm being dead serious: thank you, man," Paulsen said to close out the interview. "It was a hell of a run and you meant a lot to us."
"Well, thanks guys for having me on, and thanks for being good to me and fair to me with your interviews and stuff," Niskanen replied. "It's been fun. Goodbye for now, I guess. Thanks for everything, guys."