Castillo is slated to work out of the bullpen and will piggyback starter Bryce Miller during Sunday's game against the Diamondbacks in Seattle, Adam Jude of The Seattle Times reports.
Though both Castillo and Miller have expressed a reluctance to being used in a piggyback arrangement, the two pitchers will nonetheless operate in a tandem for a third consecutive turn through the rotation. The setup worked well the last time out Monday in Sacramento, with Castillo starting the game and delivering one of his better efforts of the season, as he struck out six and allowed just two hits and two walks over four scoreless frames. Miller followed with five innings of two-run ball and was credited with the win, but Castillo should have better odds of factoring into the decision Sunday while he and Miller are used in the opposite order. The Mariners will reassess their rotation plans after Sunday's contest, and Castillo may need another strong showing against Arizona to ensure that he avoids moving into a full-time relief role.