I think Petry is gone regardless. I know MacT says that the Oilers have long term plans for Petry but contracts speak louder than words. In interviews MacT is already using some of the same kind words he did for Dubnyk.
I believe that the narrative for progression in our D is something like this, based on contracts and clues from the people in charge of this team (I'm trying to purge my own opinions from this):
2015-16
Marincin Schultz
Nurse Fayne
Ference Niktin
2016-17
Nurse Schultz
Marincin Klefbom
Ference Fayne
Nurse will be ready after this season, and so I'm assuming that Niktin's spot on this team must be connected to Klefbom, who is a lesser prospect. Klefbom also plays RHD for OKC and I think that is by design. Regardless, I see Klefbom playing for the Oilers after the 2015 trade deadline after the inevitable Petry trade, but being sent down again after the season is done. Oddly, this will happen again with a deadline trade of Nikitin in 2016. Klefbom will come up, this time permanently. So Nurse will be a permanent full season member of the Oilers before Klefbom.
Other prospects are insurance, since it is always possible to bust out or for an unlikely player to ascend, but of course "the plan" in sports is always subject for revision.