Right now the Red Sox's first round pick is 24th and could go a few spots higher, which would be awesome. They could draft a total bust with that pick. They could also draft the next N.Syndegaard with that pick. The other important consideration: by losing their first round pick, the Red Sox would have a much smaller draft budget, and this would make it more difficult to sign an over slot prospect later in the draft (to buy out his college commitment). So, in some ways, the Red Sox would lose two prospects by signing Bautista, not one.
When you factor in Moreland's ability to prevent runs, something Bautista is no longer good at, the difference between the two is not as significant as we would normally think, especially if we factor in a decline in Bautista's production (age) and an increase in Moreland's (Fenway might augment his BA).