The Seattle Mariners and Chicago White Sox pulled off a big trade two days before the Aug. 3 MLB trade deadline.
The White Sox are acquiring starting pitcher, a three-time All-Star, according to multiple reports. The Mariners will get reliever Seranthony Dominguez, outfielder Nolan Jones and catching prospect Boston Smith. ESPN’s Jeff Passan was first to report the deal.
It’s a rare trade between two contending teams — the White Sox are leading the American League Central, while Seattle is two games out of the third and final AL wild-card berth.
The trade also resolves the season-long intrigue around the Mariners’ six-man starting rotation — a luxury for a team that has struggled to repeat its run to the 2025 AL Championship Series.
Castillo, 33, was the weak link in a rotation led by Bryce Miller, George Kirby, Emerson Hancock, Bryan Woo and Logan Gilbert. While the other five were all 28 or younger when the season began, Castillo was both the odd man out and the eldest member of the group.
According to ESPN’s Alden Gonzalez, the White Sox will take on the entirety of Castillo’s contract, which will keep him in Chicago through the 2028 season if he pitches 180 innings in 2027.
Both sides are assuming some amount of risk. Castillo last made the All-Star team in 2023. He’s 3-9 with a 5.06 ERA in 20 games (17 starts). Both his walk rate and strikeout rate, once excellent, are trending in the wrong directions.
Mariners snag reliever, promising catcher
Dominguez, 31, has also struggled after signing a two-year, $20 million contract with the White Sox in January. He has saved 12 games and struck out 28 percent of opposing hitters; he’s also blown five save opportunities and allowed home runs at twice the rate he did in 2024, when he split the season between the Baltimore Orioles and Toronto Blue Jays.
Dominguez won’t have to close for the Mariners with Andres Muñoz entrenched as the closer. The right-hander figures to slot into a set-up role, while Jones (who has spent the entire season at Triple-A) will likely head to the minors.
The most intriguing player going to Seattle is Smith, the No. 15 prospect in the White Sox system according to MLB Pipeline. The 23-year-old was buried on a depth chart that included catchers Kyle Teel and Edgar Quero.
In 22 games at Double-A Birmingham, he was slashing .306/.479/.667. Smith won’t help the Mariners this year, but he’s a well-regarded prospect — no surprise the White Sox dealt him ahead of the deadline.
The only surprise is that Smith will go to a team contending for a postseason berth in the same league as the White Sox.
