Guy has been underyped the last couple years, he's definitely demonstrated the potential to become an NHL starter. Watching him play in the NHL, a few things stand out: 1) His rebound control is great, especially compared to Chad Johnson. Most of the time, he's either able to hold onto the puck or direct it into a good area. Doesn't turn routine low-percentage chances into goals by giving up big fat rebounds like Johnson does. 2) Not sure his hips are fully recovered or rehabbed yet from his off-season surgery. Once he goes down, he looks slow. 3) He's gotten better each game so far. First game, his reads weren't quick enough, and he looked nervous/shaky. Next one, he was more solid. Last game, he made a handful of impressive stops.
All-in-all, if he can generally maintain league average numbers, there's no reason he shouldn't keep getting the bulk of the starts until Lehner returns. Unless he really takes off in that time, once Lehner comes back I think he goes back to the AHL to get more minutes and seasoning, though. Down the road, I would not be at all surprised if he ends up being Buffalo's starter.