He can't field enough to be a SS.
Neither could Prado (regarding his comp).
I have always been a big Peterson supporter, and really think both Prado and Coghlan are pretty good comps IMO. The only place Jace could probably make it as a regular would be at 2B, but unless this is his "step forward" and he can maintain something resembling this kind of production - ~.350 OBP with some gap power - he's likely to be our Coghlan-type until he reaches free-agency. Assuming Dansby and Ozzie start next season as the regular starters, Jace shouldn't put up big enough numbers to make much even in his arb years.
Neither of those guys is a perfect comp - Prado and Jace could field well enough to be an emergency SS if something happened to BOTH Swanson and Albies for a short time, and Coghlan and Jace could probably defend well enough to be an emergency CF if something happened to BOTH Inciarte and Mallex for a short time.
I really like the idea of carrying both Jace and d'Arnaud to start 2017 - they'd give us all kinds of flexibility to potentially carry both Adonis and Ruiz as a platoon at 3B if we don't go after Cespedes or Desmond and have a platoon behind the plate while still carrying an extra pen arm...
C- Wieters/???/Flowers
1B- Freeman/Garcia
2B- Albies/Peterson/d'Arnaud
SS- Swanson/Albies/Peterson/d'Arnaud
3B- Garcia/Ruiz/Peterson/d'Arnaud
LF- Mallex/Garcia/Peterson/d'Arnaud
CF- Inciarte/Mallex/Peterson
RF- Markakis/Inciarte
would only require 12 position players.