Brantly is out of Minor League options, which means he'll need to make the 25-man roster or will be eligible to be claimed off waivers.
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"He has a very good track record offensively in the Minor Leagues," manager Scott Servais said. "He's a left-handed hitter, still young with some big league time under his belt. It creates a little more depth for us there and we'll see how it plays out. We'll throw him in the mix and see what happens."
Brantly knew that his options situation left him vulnerable with the White Sox, so he's not surprised to be changing teams and hopes to take advantage of his new situation.
"You have to be excited by opportunities, no matter where they lie," Brantly said. "Of course, when I was over there with the White Sox, I would have been excited about the opportunity to break camp with them and go and chase a World Series. But now I'm ready to chase a World Series here with the Mariners."
Brantly has played 112 games in the Majors with the Marlins (2012-13) and White Sox (2015), posting a .225/.286/.317 line with five homers and 32 RBIs. His addition gives the Mariners five catchers on their 40-man roster -- joining Chris Iannetta, Steve Clevenger, Mike Zunino and Steven Baron.
Brantly hit .310 with eight homers and 38 RBIs in the Minors last year for the White Sox, but he batted just .121 in 33 at-bats in 14 Major League games after opening the season on the disabled list with a fractured left thumb.
Clevenger was acquired from the Orioles in a trade for Mark Trumbo in December as a left-handed-hitting complement to Iannetta, so Brantly figures to be competing with him for the backup position.
Cook, who signed as a free agent, has been sidelined most of the spring by a strained right lat muscle in his back. The 28-year-old pitched in one Cactus League game, a scoreless inning against the Padres in Seattle's spring opener, before being shut down.
"He's going to be down for a significant time, so it creates a little roster flexibility and we were able to get Brantly on board," Servais said. "It's too bad for Cook. He was throwing the ball really well early, but stuff happens. I'm anxious to get him back. I don't know when that will be."
Cook posted a 2.77 ERA in 208 relief appearances during his first four Major League seasons from 2011-14 and was an American League All-Star in 2012 for the A's, but he struggled last year and had an 18.69 ERA in niine relief appearances for Oakland and Boston.