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.
We're buyers. We're committed to winning. That's what [owner Vinnie Viola] has said all along and what we've said all along. We want to win. We want to win a championship. We're putting the pieces together, step by step, and we're hopeful that Jaromir can lead these young guys to the promised land. It's a win-win for us.
— Panthers GM Dale Tallon on Jaromir Jagr, who he traded for from the Devils on Thursday