In Zachary Levine's post on tonight's game, Mills gave a clue as to who might get sent down when Jed Lowrie is activated on Friday (presumably):
Mills:
“I like having an eight-man bullpen, but I don’t like the strappings it puts on the ballclub in other areas. What we have now is probably an eight-man bullpen for the first six games and then we’ll see how we go from there.”
It's far too early to speculate on who it might be (hint: it won't be Rhiner Cruz), but the bullpen is going to be competing for a job over the next five days.