I don't think they'll replace Abraham with this "Roy Stark". They're keeping the backgrounds of the comic characters in check. Abraham didn't have a medical background, but if I remember correctly, he was military. Even the guy who plays Tyreese on Talking Dead said that his character played in the NFL and they didn't say anything about that on the show.
I'm thinking this "Roy Stark" may be a created character like Daryl or Merle and may be an amalgam of several comic characters that won't make an appearance.
The more I've thought about it, the more I think that the show is still on schedule to follow the arch set by the comics. The Woodbury people DID refuge in the prison, but that was after Rick's gang was severed in different directions.
With the Governor leaving with his two main henchman, he could discover Woodbury is abandoned and find the biggest weapon he was missing from the comic raid on the prison:
He returns to **** **** up and the story arch returns to normal.
I'm anticipating a lot of flux in season four. Stability should be at a premium and they should be on the road for a vast majority of it.
The main thing that people have said about the TV version of the Governor is that he's tame compared to his comic counterpart.
I think now that he's done what he's done, we got the visual of how the governor became who he is, rather than the mad hatter we were introduced to immediately in the comic.