Long-awaited Luis Castillo trade delivers everything Mariners could have asked for

After literally years of rumors and speculation, Luis Castillo is finally an ex-Seattle Mariner. He's headed to the Chicago White Sox by way of a trade on Saturday that accomplishes two things the Mariners badly needed to do: lighten the payroll and deepen the roster.

The deal was first reported by ESPN's Jeff Passan, who has the return as right-handed reliever Seranthony Domínguez, outfielder Nolan Jones and catching prospect Boston Smith. Crucially, the Pale Hose are also taking on Castillo's remaining contract, which is valued at roughly $30 million even without the inclusion of a vesting option for 2028.

And so, Castillo's run with the Mariners is over. It began four years ago when he arrived via a blockbuster trade with the Cincinnati Reds on July 30, 2022. Many good times followed, including a dominant role in the 2022 club's snapping of a 21-year playoff drought and a fifth-place finish in the AL Cy Young Award voting in 2023.

The record will show that Castillo's final act as a Mariner was to post a 5.06 ERA over 99.2 innings, with a career-low -0.7 rWAR. At least he went out on something of a high note, with a 4.11 ERA over his last 12 outings.

Seranthony Domínguez might be the reliever the Mariners needed, and he's not coming alone

It was only a few weeks ago that Mariners president of baseball operations Jerry Dipoto said that there could be buyer-to-buyer trades ahead of the August 3 deadline. And that's exactly what we have here.

Whereas Castillo is the workhorse starter the White Sox badly needed, Domínguez is the late-inning reliever that Dan Wilson needed in his bullpen. Or so the Mariners hope, and that indeed requires looking at his 4.10 ERA and trusting there's more to like about the 31-year-old righty's season.

It's not the easiest sell, in part because Domínguez is walking over 5.0 batters per nine innings for a second year in a row. Yet he's also allowed just a .201 average across the last two seasons, and that largely traces back to a legitimately nasty arsenal. He averages 97.3 mph on the four-seamer, and his 27.8 K% is in the 84th percentile.

At the very least, Domínguez figures to be a useful tool for Wilson when the task at hand is taking down a string of right-handed batters. Ideally, he'll continue a hot stretch that has seen him allow one earned run in his last nine appearances.

There's about $14.6 million remaining on Domínguez's two-year, $20 million contract, so the financial savings here are legit for the Mariners. If the idea is to keep buying after this trade, that flexibility can only help.

Just three years after earning NL Rookie of the Year votes with the Colorado Rockies in 2023, Jones hasn't played in any MLB games this season. He looks like a throw-in, whereas Smith had been ranked as Chicago's No. 15 prospect by MLB Pipeline. And that may undersell his standing, as he has a 1.037 OPS and 22 home runs to show for 78 minor league games.

To hear it from Baseball Trade Values, this trade rates as a major overpay by the White Sox. That's easy enough to believe without a fancy formula, as it feels as if Dipoto has somehow sold high on a depreciated asset. As such, he's now set up to win not one, but two Luis Castillo trades.

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