The Mariners got a great buy-low opportunity on a prospect by picking up Gaby Hernandez for lefty Arthur Rhodes. Hernandez works with a solid-average fastball, touching 94 mph, with a great delivery that puts good downhill plane on the heater. He throws a sharp slider-cutter with good tilt that he'll run down and in to left-handed hitters, while his curveball has good downhill plane but is far less consistent.
Hernandez has always had good command and control, but his performance this year in AAA was atrocious as he posted a 7.24 ERA in Albuquerque before Florida sent him back to AA earlier in July. Albuquerque is one of the best hitters' parks in the minors, perhaps the best overall considering the league, and Hernandez was pounded at home, giving up 61 hits in 38 innings in Albuquerque this year. Tacoma is a far better pitching environment than Albuquerque, and Safeco is also a big ballpark that kills right-handed power. I could see Hernandez's stuff and command playing up in both of those situations.
To get that kind of prospect, even considering his poor performance this year, in exchange for a situational lefty who probably wouldn't throw more than 15 innings the rest of the year for Seattle is a minor steal.