Buxton is far from a lock. He might be leading centerfielders in WAR (technically Julio Rodriguez has passed him in bWAR), but they vote on outfielders as a whole. He's only 7th in the AL among outfielders - Bader has even snuck by him for the moment. And assuming he'll pick up right where he left off post-concussion is pretty optimistic. He's had a great stretch, but he definitely has some work to do.
I think Ryan has the best chance on the roster of getting picked before the manager-fill-out-the-roster phase, and that stage becomes a crapshoot based on which teams need someone and who their viable candidates are.