Callahan, 22, made his NHL debut on March 25 against the Columbus Blue Jackets. He was second on the Grand Rapid Griffins of the American Hockey League with 26 goals and fourth with 44 points in 70 regular-season games.
Callahan was a sixth-round pick (No. 180) in the 2009 NHL Draft.
Nestrasil, 23, had 16 goals and 36 points in 70 games for the Griffins. He was a third-round pick (No. 75) in 2009.
It's not always going to be pretty, especially against a great goalie [Lundqvist] like that. My goals are never how you draw them up. It's just a matter of grinding it out and trying to create havoc in front and good things happen.
— Capitals forward Jay Beagle on scoring in their Game 3 victory against the Rangers on Monday