The Mariners reassigned Castillo to minor-league camp Monday.
After spending the past two seasons in Japan's Nippon Professional Baseball, Castillo returned stateside over the winter, joining the Mariners on a minor-league deal that included an invitation to spring training. The 30-year-old didn't mount much of a case for a spot in Seattle's Opening Day bullpen, giving up three earned runs on nine hits and two walks while striking out five over five innings across his three Cactus League appearances. Castillo is expected to report to Triple-A Tacoma to begin the 2025 season.