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Iwakuma has gone 47-25 with a 3.17 ERA in four seasons with the Mariners, posting a strikeout rate of 7.6 and a walk rate of 1.7 while averaging 163 innings per season.
The 34-year-old gave up 13 runs (12 earned) in 16 1/3 innings in his first three starts of 2015, then missed seven weeks with a strained lat. Upon returning in early July, Iwakuma posted a 3.10 ERA in his final 17 starts, finishing the season with a 3.54 ERA over 129 2/3 innings.
Iwakuma -- an All-Star who finished third in American League Cy Young Award voting in 2013 -- will enter a star-studded free-agent class highlighted by fellow starters David Price, Zack Greinke, Jordan Zimmermann, Johnny Cueto and Mike Leake.
Alden Gonzalez is a reporter for MLB.com. Follow him on Twitter @Alden_Gonzalez and Facebook, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.