I think that when the medical team say a player is ready, he probably is.
Now, it's also a fact that after such a long absence, the situation is a bit tricky. I trust Hierro though, I think his experience and dependability are valuable. I'd much rather have him rather than anyone else (given the absences of Salgado and Karanka we'll have anyway).
From what I hear and read he'll start for sure.