Mariners trade Luis Castillo to White Sox for bullpen arm and prospects

Seattle traded right-hander Luis Castillo to the Chicago White Sox in a deadline move that brings reliever Seranthony Domínguez and three minor-league prospects to the Mariners. The White Sox will assume the remainder of Castillo’s contract, including part of the $22.75 million due in 2026 and the full $22.75 million for 2027. Domínguez, a 31-year-old set-up man, carries about $12 million in salary through the 2024 season. The deal saves Seattle roughly $18 million in guaranteed payroll, providing additional flexibility for the upcoming offseason.

Domínguez has posted double-digit hold or save totals in each of his six full seasons, though his walk rate has risen and his sinker usage has declined, leading to higher expected batting averages against. He remains effective in strikeout rate and whiff percentage and is expected to fill a middle-relief role with potential to assume higher-leverage duties.

The three prospects include outfielder Boston Smith, who has advanced rapidly through the minor leagues and posted a wRC+ of 196 at Double-A, and two other players whose specific details were not disclosed. Analysts note that while none of the prospects are projected to make an immediate impact at the major-league level, they add depth to Seattle’s farm system.

Mariners general manager statements were not included in the report, but the transaction is viewed as a financial and roster-management decision that could allow Seattle to pursue additional moves before the season’s end.