Luis Castillo is on the move, as the Mariners are set to trade the veteran righty to the White Sox, per reports. In exchange, the White Sox are surrendering over Seranthony Domínguez, plus Minor Leaguers Nolan Jones & Boston Smith.

The White Sox’s Return

• SP Luis Castillo (5.06 ERA, 86 K over 99 IP)

The Mariners acquired Luis Castillo in the 2021-22 offseason with the idea that the righty, at the time a dominant force with the Reds, could be just that for Seattle. And he was, as Castillo was a major driving force behind the Mariners cracking the playoffs in 2022 for the first time in 20 years.

Unfortunately for the Mariners, Castillo’s production gradually dipped year over year. After a career-best 219 strikeouts during the 2023 campaign, Castillo’s K/9 dipped every year between 2024 and 2026. And if the season were to end today, Castillo’s 5.06 ERA would be a career-low for the veteran right-hander.

It’s been an interesting year for him, as Castillo began the season in the rotation, then worked in a piggyback role (to some conflict with the club) with Bryce Miller, and most recently, was back in the rotation as of late June.

As far as tendencies are concerned, Castillo gave up a lot of contact & base hits with Seattle this season. Far more contact compared to past years. On top of that, Castillo, who’s used his slider as his primary secondary offering against right-handed hitters, saw significantly fewer swing-and-misses of the slider.

Now, it is a pitch that Castillo will look to use against lefties, not much of a shock given that so much attention nowadays is towards getting hitters off the fastball. Castillo, who sat 95-97 MPH with both the two-seamer and four-seamer, gave up six home runs off the slider this year, more than in all of 2024 (5) and 2025 (3).

And among those six, Castillo hung them either in the happy zone for lefties (see below) or right over the heart of the plate.

Castillo has one year remaining on his deal at $24.15MM. Additionally, Castillo possesses a $25MM vesting option for the 2028 season.

The Mariners’ Return

• RP Seranthony Domínguez (4.10 ERA, 45 K over 37.1 IP)
• 1B/OF Nolan Jones (.255/.362/.442, 14 HR & 31 XBH over 91 GP (AAA))
• C/OF Boston Smith (.291/.448/.589, 22 HR & 32 XBH over 78 GP (A, A+, & AA))

Seranthony Domínguez’s tenure in Chicago didn’t last long.

Domínguez, who signed with the club back in January, was expected to be the club’s closer heading into 2026 after a good, yet somewhat dramatic stint with the Blue Jays during the 2025 playoffs. A proven hand with playoff experience & with a new splitter in tow, Dominguez netted himself a lot of swing-and-miss last season.

However, things were interesting for the right-hander. Domínguez couldn’t retain the closer’s job for long, as struggles in the role, coupled with the rise of Grant Taylor, created a perfect storm for him to lose said job.

Like with many hard-throwing relievers — and Seranthony is one of those — it all comes down to execution. More swing-and-miss on that splitter this season. However, his fastballs were getting tagged, and, as you’ll see further, that wasn’t the only problem for the reliever.

He also allowed six home runs on the season, although just one of those came after May 24. That, though, was offset by 12 walks over the last 14.2 IP for the club.

Nolan Jones spent 2026 split between the Guardians & the White Sox organization. Jones was acquired by Chicago for future considerations back in June of this year. The book is pretty clear on him: lots of pop but also swing-and-miss.

The other piece in this deal is Boston Smith, a sixth-round pick from 2025 who’s in the midst of a breakout season in the Minors. Smith wasn’t even a White Sox draft pick. Chicago acquired him earlier this spring for Curtis Mead, who is now a Red Sox.

While Mead had his first real big-league success in 2026, Smith proceeded to hit 22 home runs to this point this season, which earned him two in-season promotions. Those who’ve read our AL Central farm reports (see June & July) have a pretty good idea of what he can do. Lots of pop, particularly to the pull side, plus a high tendency to take pitches & work counts.

The swing-and-miss was high for Smith; he had a 34% Whiff% in his first few weeks in Double-A Birmingham.

Smith had a 24% CS% rate as a backstop. He also got looks in the outfield, with fourteen games in left this season.

Analysis

So is the life for teams that are both in it — but also keeping a mind towards 2027 & beyond.

The Mariners, amidst a potential payroll crunch thanks to looming free agency for several of their young pitchers, plus pending free agent Randy Arozarena, were able to move off a big contract in Castillo. Seattle had to take on a contract with Seranthony Domínguez, a reliever who is good when on.

As for the White Sox, Chicago adds reinforcements in a year where the White Sox very much have a shot at winning the AL Central. The club’s rotation has been very good, as the likes of Sean Burke, Davis Martin, as well as Erick Fedde & Anthony Kay.

However, filling out that fifth spot has been somewhat of a pain. Noah Schultz has gone through his growing pains as a rookie in an injury-shortened year.

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