I don't think Santos is done for the season, but he'll be out for awhile, and even when he comes back, I don't think he will be as effective.
While I agree that Perez has been absolutely dominant as a reliever, I think the 40 innings or so that he would log as a closer wouldn't be his most valuable contribution.
Frankly, I'd rather see him as a starter, as I suspect that we're going to need some help in that department. I don't see Alvarez or Hutchison lasting the season.