Castillo has struggled in 2026, posting a 5.06 ERA over 99.2 innings.
With the Mariners featuring a pitching staff with more than five starting candidates, Castillo had been relegated to bullpen duty three times this year. In those outings, he would work as the second half of a piggyback approach with another starter.
The 33-year-old is under team control through the 2027 season and has a vesting option in his contract for 2028, which would vest if he throws 180 innings next year.
According to multiple reports, the White Sox are taking on the remainder of Castillo’s contract, which includes roughly $30 million over the next two seasons.
Seattle will look to get the best out of Domínguez, who lost the closer job on the South Side earlier this season and has posted a 4.10 ERA and 45 strikeouts in 37.1 innings. He has also recorded 12 saves.
In 2025, the 31-year-old carried a 3.16 ERA while splitting the year between Baltimore and Toronto.
Also heading to the Mariners are Jones, a 28-year-old who has spent the entire season in triple-A and owns an .807 OPS, and Smith, a 23-year-old catcher who has 22 home runs on the season in 78 games at high- and double-A.
