Mariners get Domínguez for 'pen help in Castillo trade with White Sox

SEATTLE -- The Mariners are on the board in this Trade Deadline season, and they made the move that they’d hoped to make all along.

Seattle is sending veteran right-hander Luis Castillo to the White Sox, the club announced on Saturday night, shortly after multiple sources confirmed to MLB.com that it was in the works.

In doing so, the Mariners both netted bullpen help -- via veteran right-hander Seranthony Domínguez -- and even more notably, are not sending any money on the remaining $30 million guaranteed to Castillo through next season, a source said.

Castillo also has a $25 million vesting option for 2028 if he accrues 180 innings in ‘27.

TRADE DETAILS
Mariners receive: RHP Seranthony Domínguez, OF Nolan Jones, C Boston Smith (SEA No. 14 prospect), cash considerations
White Sox receive: RHP Luis Castillo

Moreover, the Mariners also received money from the White Sox to account for Domínguez, who signed a two-year, $20 million contract in January that includes a $2 million buyout on a $12 million mutual option for ‘28. He’s earning $7 million this season and $10 million in 2027.

Leading up to Monday’s 3 p.m. PT Deadline, the Mariners had made it clear across the industry that they were willing to deal from their surplus of starting pitching.

And among their six starters, Castillo had long been their preference to move -- given his age (33) and that he’s their most expensive player ($24.15 million annually this season and next).

But for those reasons, Castillo also presented the most complicated case.

Even so, Seattle’s front office is still open to engaging clubs on another potential trade from its rotation, a source said.

They’d also been actively listening on Emerson Hancock -- and as of Saturday morning, they’d been more aggressively approached about George Kirby.

Regardless of how or when his Mariners tenure ended, Castillo has been a defining piece to Seattle’s success of this era -- and his acquisition from the Reds in the 2022 Deadline blockbuster remains one of the biggest trades in franchise history.

It wouldn’t be a stretch to say that transaction represented the moment that their rebuild ended, as he immediately ascended to the top of their rotation and helped the club snap a 21-year playoff drought.

Now in his 10th season, Castillo has lost a bit of that elite stuff -- but he’s still remained a workhorse, logging at least 175 innings and 30 starts from 2023-25, his three full seasons in Seattle.

That said, he’s experienced more volatility in ‘26, with a 5.06 ERA and 78 ERA+ (league average is 100) in 20 outings (17 starts). Some of those numbers were compounded by a rough, nine-start stretch to begin the year, which led to him being piggybacked with Bryce Miller for four starts from mid-May to mid-June.

Part of the calculus to do so, instead of moving Castillo to the bullpen permanently, was to keep him stretched out for this precise moment -- as his trade value would’ve tanked had he been in a reliever’s routine. There’s also the fact that he was the Mariners’ most expensive player and the pitching staff’s elder statesman.

That the Mariners had been more open to deal from their rotation has represented a philosophical shift, as they had adamantly avoided dealing from that group since emerging from their rebuild late in 2021.

But with their top prospect Kade Anderson expected to be part of their permanent plans as soon as 2027, and potentially sooner for a playoff push, and No. 2 prospect Ryan Sloan not far behind, they were willing to deal from their big league depth to bolster the rest of the roster.

Domínguez, 31, is in his eighth big league season, and has a 4.10 ERA (105 ERA+) in 42 outings. The Mariners had been actively seeking bullpen help, and he’ll slot into a high-leverage role leading up to closer Andrés Muñoz.

Mariners fans will remember Domínguez for giving up the grand slam to Eugenio Suárez in Game 5 of the 2025 American League Championship Series, when he was with the Blue Jays.

Jones, 28, represents outfield depth in the upper Minors but could be a compelling piece, having finished fourth in National League Rookie of the Year Award voting in 2023 after a 20-homer, 20-steal season with Colorado. But since then, he’s been worth minus-1.3 wins above replacement, per FanGraphs, having spent ‘25 in Cleveland and not appearing in the Majors in ‘26.

As for Smith, this is the second time he’s been traded this season -- having been acquired by the White Sox earlier this spring from the Nationals for infielder Curtis Mead, who was recently sent to the Red Sox.