Including three times with the New York Rangers (2012, 2013, 2015), Hagelin faced the Capitals six times in the playoffs. He was on the winning team in the first five series before the Washington defeated Pittsburgh in six games in the Eastern Conference Second Round last season and went on to win the Stanley Cup for the first time.
In a strange way, the familiarity from so many postseason matchups has helped Hagelin feel comfortable quickly with the Capitals. He has three points (two goals, one assist) in eight games with Washington, including the winning breakaway goal in a 3-1 victory against the Winnipeg Jets on Sunday.
His two goals with the Capitals equal his combined output from 38 games this season with the Penguins and the Kings (two goals, six assists).
"It's been good," Hagelin said. "I started feeling better in LA coming back from my injury, and once I got here I had some confidence coming in and obviously there's a mutual respect between the players on this team and myself from playing against each other a lot. So, it wasn't really a hard transition."
Hagelin's speed and tenacity have had the positive impact the Capitals envisioned on their forecheck and penalty kill, which has killed off 16 consecutive power plays over the past five games, including five against the Jets.
"He's clutch and he's been huge for us on the penalty kill," Capitals forward Tom Wilson said. "His skating is elite and we're happy to have him on our side."
Hagelin remains in touch with some of the Penguins, including Crosby, Kessel and good friend, Patric Hornqvist. He imagines, "It will be weird for them," more than it is for him on Tuesday.
He said facing the Penguins in the Kings' 5-2 win at Los Angeles on Jan. 12 helped get some of the awkwardness out of the way for him.
"It's definitely different to see him on the other side but I wish him the best," Penguins forward Jake Guentzel said. "Just to see him over there is going to be weird feeling, but I really liked the person."
NHL.com correspondent Craig Merz contributed to this report.