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He's got 2.5 more years left on his club control (FA in 2020), is consistently one of the lowest hits-allowed guys in the league, career FIP of 3.60 and profiles as a really quality replacement for Iwakuma who has had some ace-level stretches in him.
The As are said to want legit prospects. The Mariners have an outfield full. Would you be willing to part with top-5 Ms prospects O'Neill and Neidert (plus some low-minors extras) for Gray and his career 113 ERA+ despite the injury concerns from last year? It's not what Quintana brought back but it's about as close as the Ms are likely to get.
Good article on Gray from Sullivan, btw: http://www.fangraphs.com/blogs/sonny-gray-is-back-and-he-isnt/