Yes, Ranger is demonstrably suited to bullpen work, as well as to a flex role. It would be curious to limit him in that way given how excellent he was to start the season, let alone squander assets trying to do better, but neither his excellence nor his durability is a sure thing right now.
And Sanchez has also spent most of his MLB career as a reliever. Not a memorable one, but he can do it. And will, unless he's left off the NLDS roster entirely.