To define an RP I will use two sorts to limit the field.
Durability greater than 50, control greater than 50.
I am not saying you cannot find candidates who would work but don't meet the criteria but the players with Dur under 50 won't be able to pitch often and the ones with control under 50 are going to walk too many people for a relief role and waste too many pitches on balls.
So who is it?
Carlos Espinosa has a stamina too low to be more than a spot guy. His ratings are good, except for that 39 third pitch.
Jimmie Batista has been successful historically and has decent pitches. His availability may be limited but he should be solid.
Garrett Day has had a Bret Saberhagen like career. This should be the good season.
Type A performer Alex Richard will get signed eventually. He is good and still has years in him. At this point he is cheap too.
I almost left off Artie Bush because his stamina of 6 will limit his IP. He looks useable but not reliably.