In 19 games with the Flyers, MacDonald has four assists and has helped upgrade the play of his defensive partner, Luke Schenn. He also has helped reduce the minutes - and some wear and tear - on veteran Kimmo Timonen.
"It was just a matter of trying to see if you fit in, and I felt like I fit in really well," MacDonald said. "It's a great group of guys in here, and I'm really happy to make the commitment."
Flyers general manager Paul Holmgren said his goal was to sign MacDonald and "get it off his plate and let him focus on playing in the playoffs."
The smooth-skating Gostisbehere (pronounced GOSS-tis-bear), who led Union College to the NCAA championship Saturday at the Wells Fargo Center, bypassed his senior collegiate season. He will report to the Phantoms on Thursday and play in Adirondack's final two AHL games over the weekend on an amateur tryout contract.
The 5-foot-11, 170-pound defenseman will play with the Phantoms on an amateur tryout contract this season and then will join the Flyers for their practices. He is not eligible to play in this year's playoffs.
Holmgren said it was "not far-fetched" that Gostisbehere, who will turn 21 on Sunday, will play for the Flyers next season.
But he probably will need some seasoning with the Phantoms.
"He's an exciting young player," Holmgren said. "It's going to be a big summer for him to continue to get stronger and prepare himself for the rigors of an 80-some game schedule, or thereabouts, depending on where he plays."
The Flyers want Gostisbehere - a third-round draft pick in 2012 who is nicknamed "Ghost" - to put on some weight, but not so much that it will reduce his speed.
"You don't want to load down your Ferrari," Holmgren cracked.
Asked whether he was ready to play in the NHL, Gostisbehere said, "At this point, I don't even know. I haven't even played a professional game yet. Whatever they want me to do, I'm all ears. I'm just happy to be in this situation."
Lines in focus
Based on Tuesday's practice, wingers Michael Raffl, Jason Akeson, and Zac Rinaldo will be on the second, third and fourth lines, respectively, in Game 1 in New York on Thursday.
Akeson, 23, was called up from the Phantoms for the last regular-season game, and Berube liked what he saw.
Excluding the injured Chris Pronger, the Flyers have about $22 million in salary-cap hits committed to five defensemen next season. . . . Winger Steve Downie missed practice with an unspecified injury, and center Vinny Lecavalier was given the day off.