Jason Bay is a bad fit for the Mariners defensive philosophy. I would assume that they would get him because they feel like Gutierrez can cover a lot of his range deficiencies.
My guess is that he will field DH much better than whoever fielded that position last year.
The Ms really don't need great starting pitching beyond King Felix. They can resign Jarrod Washburn and make him look good with that big outfield and guys who chase flies.
Fenway tends to make left fielders look worse on UZR due to the wall. The wall tends to rob LFs of a lot of plays that could otherwise be made. They have an adjustment, but it still is not quite right. It's known as the Manny Effect. I don't know if there is something similar with respect to +/-.