I'm not sure either, but with Burnett gone and Jones a high draft pick from last year I'm thinking he gets first shot at playing SS full time, or at least as full time as anyone is in these days with all kinds of packages and hybrid roles.
With the way the game is subtly changing, it could be that Burks is being looked at as a starting/full-time ILB even though he is just a "bit" undersized. I suspect Ryan's snaps may be going down even more next year. Burks ran quite a bit faster 40-time than Martinez as well, so it could be once he gets settled in (if he gets settled in?) that he takes snaps from Martinez as well on passing downs.
I agree that Pettine is probably influencing things some. Even if he wasn't, it was clear to everyone, even amateur hacks like me that this team lacked speed and coverage ability. The picks so far are designed to fix that I think.