Seattle Mariners Trade Starting Pitcher to Chicago White Sox

Hours after the Seattle Mariners' crucial 4-3 win against the Minnesota Twins on Saturday, the former made their first big move of the trade deadline and sent away one of the most respected veterans on the team in the process.

Per a post on "X" from ESPN MLB insider Jeff Passan that has since been corroborated by several sources, the Mariners dealt starting pitcher Luis Castillo the Chicago White Sox in return for outfielder Nolan Jones, right-handed reliever Seranthony Dominguez and catching prospect Boston Smith.

Per a subsequent report on "X" from ESPN reporter Alden Gonzalez, the White Sox will take on the remainder of Castillo's contract, which includes the prorated amount on his $22.75 million salary this year, his $22.75 million salary in 2027 and the potential $25 million vesting option he has for 2028.

Seattle originally acquired Castillo in a trade with the Cincinnati Reds at the deadline in 2022 and he helped the team to break its 21-year playoff drought that season. The Mariners signed Castillo to a five-year, $108 million contract extension, which runs through '27, not including his vesting option.

The 33-year-old has proved to be a steady and veteran voice in a Mariners rotation that is otherwise all comprised of drafted and homegrown talent and still on their first major league contracts. He made the All-Star Game once for Seattle in '23 after two career nods to Midsummer Classic with Cincinnati.

However, the three-time All-Star has struggled this year. He's posted a career-worst 5.06 ERA and has struck out 86 batters in 99.2 innings pitched across 20 appearances (17 starts). When fellow starter Bryce Miller was activated off the injured list, the Mariners opted for a piggyback approach between him and Castillo with the two rotating between getting starts and pitching in long relief. The M's eventually went to a traditional six-man rotation, which is now down to a typical five with the deal.

As for the players Seattle will get in return, Dominguez is on the first year of a two-year, $20 million deal and is earning $7.5 million this season. He has a $10.5 million salary for 2027 and a $12 million mutual option for 2028.

Dominguez has posted a 3.99 ERA this year and has fanned 47 batters in 38.1 innings pitched across 43 outings. He has 12 saves in 17 opportunities and gives the Mariners another high-leverage/late-inning option out of the bullpen.

Jones has spent the entire season in the minor leagues and was traded by the Cleveland Guardians to Chicago on June 11. In the minor leagues, he's slashed .255/.362/.442 with an .804 OPS in 91 games and has hit 16 doubles, a triple and 14 home runs with 59 RBIs this season. He was originally awarded a $2 million salary in his first year of arbitration, which is buried due to him spending the entire season in the minor leagues.

Smith (No. 15 White Sox prospect, per MLB Pipeline) was originally picked by the Washington Nationals in the sixth round of the 2025 MLB Draft out of Wright State. He was traded to Chicago on March 28. He was promoted to the Double-A Birmingham Barons on July 1.

In 22 Double-A games, Smith has slashed .306/.479/.667 with a 1.146 OPS and has hit two doubles and eight home runs with 18 RBIs. This year, his first in the minor leagues, he has an OPS over 1.000 and has hit 22 homers with 58 RBIs in 78 games across three levels: Single-A, High-A and Double-A.