The Mariners acquired the right-hander from Cincinnati on Friday, hoping to bolster their rotation as they chase their first playoff berth since 2001. Their postseason drought is the longest in the four major North American professional sports.
At 55-48, Seattle is 12 games behind Houston in the American League West standings but is in second place for one of three wild-card playoff spots. Manager Scott Servais said Sunday that Castillo will start the series finale in New York.
Castillo's last start for the Reds came Wednesday. The 29-year-old was 4-4 with a 2.86 ERA for last-place Cincinnati this season. He has 90 strikeouts and just 28 walks in 85 innings.
His strong performance comes a year after he had a career-worst 8-16 record, in which his losses were the most in the National League, and had a 3.98 ERA.
The Reds got infielders Noelvi Marte and Edwin Arroyo and right-handers Levi Stoudt and Andrew Moore in the trade. Marte was the Mariners' top-rated prospect, Arroyo was third and Stoudt fifth.