I doubt Rathbone even sniffs the NHL this year, unless they're just hammered with injuries. The Penguins are actually super deep on D right now in terms of "guys who can survive in short stints in the NHL". Beyond their regulars (Letang, Karlsson, Graves, Pettersson, Joseph and Ruhwedel), they have Ludvig and Shea as extras in Pittsburgh and Smith, Fedun, Ouellet, Butcher (currently injured) and Hajek (AHL deal) all in the AHL.
Rathbone is like #13 on the Penguins depth chart in terms of defensemen. He'd only be called up if the Penguins lost one of Karlsson or Letang and both Smith and Butcher would be called up to fill those roles before Rathbone would.