Mariners manager Dan Wilson said that Castillo will start Monday's game against the Athletics in Sacramento and will be used as part of a tandem with Bryce Miller, Ryan Divish of The Seattle Times reports.
In his last appearance Tuesday against the White Sox, Castillo pitched in relief for the first time in his 10-year big-league career, covering 2.1 frames and allowing two earned runs behind Miller, who started and worked the first 5.2 innings. The two will be deployed in the opposite order for the upcoming turn through the rotation, and while Castillo should have a higher innings floor as a starter, his poor form of late could result in Wilson leaning on Miller to handle the larger share of the pitching workload. Though the Mariners are taking a fluid approach to the back end of the rotation, Castillo may need a positive showing against the Athletics to avoid a demotion into a more traditional bullpen role. He'll head into Monday's start with a 6.41 ERA and 1.55 WHIP on the season, with both ratios being the worst marks of his career by a considerable margin.