"He's that character guy that really is willing to sacrifice anything for the betterment of the team," Lightning captain Steven Stamkos said of Callahan. "You can't have enough of those guys."
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The Lightning have several them, with the list including Callahan's fourth-line linemates Cedric Paquette and Chris Kunitz, but arguably nobody wears the heart and soul badge of the team more prominently than Callahan.
He made his impact on the scoresheet in Game 5 of the Eastern Conference Final at Amalie Arena with a goal and an assist on Paquette's goal in the Lightning's 3-2 win, which put them on the verge of reaching the Stanley Cup Final for the second time in four seasons (2015).
The Lightning lead the best-of-7 series 3-2. Game 6 is at Capital One Arena on Monday (8 p.m. ET; NBCSN, CBC, SN1, TVAS).
Callahan's impact has also been felt off the scoresheet.
He's Tampa Bay's leader in hits in the Stanley Cup Playoffs with 53, putting him seventh in the NHL and fourth among forwards entering play Sunday. He has 11 blocked shots, tied for second among Tampa Bay's forwards.
Callahan is one of the Lightning's leading penalty killers, averaging 1:24 of shorthanded ice time per game. If they have to kill a 5-on-3, he's out there. If they're protecting a lead in the final 90 seconds of a game, as they were Saturday, he's out there.
Against Washington, his line has been playing against Ovechkin at 5-on-5. Callahan, Paquette and Kunitz have not been on the ice for a 5-on-5 goal against in three straight games. It's no coincidence the Lightning won all three.
"He plays the game as honest as you're going to find," Stamkos said. "He plays it clean. He plays it hard. He's not one of those guys who's going to go out there and chirp your ear off. He'll do it by constantly finishing his checks, by constantly being on the right side, by constantly being in the shooting lane and blocking your shot. He's an unbelievable professional and he can change the momentum of a game, which can change a series."