I think Holt will still be the main super-sub, just that when he's not subbing in he will have a position available to him.
With Castillo, it was about spending $$$ on a prospect/lottery ticket. If it worked out, great. If it didn't work out, at least they didn't spend all that time training him and it's just money anyway. Basically, they were saying "we don't have ANY OF prospects right now, we need to pay to get one in here. This guy has a high upside and is athletic enough that he could be replacement level at the very least."