The Seattle Mariners made their first move of trade deadline season, dealing Luis Castillo to the Chicago White Sox. Kate broke down the details last night after the news broke. The initial sentiment has been positive, but also a little emotional. Luis Castillo quickly became a beloved figure in Seattle since they acquired him via trade in the summer of 2022. He lived up to his nickname, La Piedra (The Rock), missing nary a start over the last four years and exemplifying consistency. From 2023-2025,
he made 33, 30, and 32 starts, with ERAs of 3.34, 3.64 and 3.54.
Even before his disappointing 2026, trading Castillo at this deadline seemed like a foregone conclusion. The Mariners currently have 6 starting pitchers on the roster, with Kade Anderson and Ryan Sloan pounding the ground outside the door Donkey Kong-style.
Given that public knowledge, the haul they were able to get in return for Castillo is impressive. What I like most about this trade is that the Mariners aren’t relying on higher percentile outcomes for their new acquisitions for this trade to be a win.
Seranthony Dominguez – MLB RP
The “right-now” part of this trade, 31-year-old Seranthony Dominguez, will hope to add consistency to a prince-and-pauper bullpen. Dominguez has been a solid set-up man in the past, hitting double-digits on holds or saves in each of his six full seasons.
Kate noted the walk problem that has plagued him since the start of last season, but there’s another glaring problem for Dominguez — his sinker is a real stinker. Teams seem to have picked up on it, as he’s thrown it less and less. In 2022, his sinker usage peaked at 32.1%. It slowly slid down the ranks of his arsenal, bottoming out last year at 13.4%, and around 15% this year.
Despite sitting around 97 mph with acceptable movement, the sinker has been hit hard in the past, and this year, it’s getting annihilated. Here’s some numbers that should make your eyes pop: xBA: .406, xSLG: .826, xwOBA: .562, Hard-Hit%: 82.4%.
These numbers are obviously outlandish. He’s a reliever, so the sample size, she small, but the pitch has been his worst pitch for years now. And yet he throws it!
The sinker has never been that great for him, but especially since introducing his sweeper, hitters have been teeing it up. Typically, the sinker/sweeper combo is an effective combo – the spin axes and movement tend to be opposed, making it difficult to tell which pitch is which out of the hand. Compare the spin axes for the sinker/sweeper of Bryan Woo:
And Dominguez:
It doesn’t seem like a whole lot, but it’s enough to lose that extra edge of deception and make the sinker a pitch you can sit on.
He’s also tended to groove it right in the Q-zone, leading to plenty of this:
Considering one of his pitches is essentially an auto-crit for opposing batters, his overall results have been live-with-able. He’s run an xFIP of 4.03, and still ranks highly in K% (27.8%) and Whiff% (29.4%), so there’s plenty to like here with a change of scenery and a new approach to sequencing and pitch-mix. His 4-seamer still has plenty of life, and his sweeper is an improved version of his already-quality slider. I would expect him to slot in the middle of the bullpen hierarchy, with some upside to become a worthy high-leverage pick down the home stretch.
Nolan Jones – Triple-A/MLB OF
Prospect sickos might remember Nolan Jones as a former highly-regarded draft pick of Cleveland back in 2016. His power looked to be a carrying tool, reaching as high as a 70 future-grade on his raw power. Rockies fans might remember the 3.8 fWAR season he put up in the friendly confines of Coors Field back in 2023, his first 20-20 season. Some might even remember that he’s a left-handed bat who doesn’t particularly hit lefties all that well, an area that the Mariners notably struggle with.
Worry not, dear reader, Jones is not intended to fix this problem. Despite his pedigre, illusory past success and his undeniable phenotype of Dipoto-catnip, Jones should hopefully not figure too much into the Mariners’ 2026 season. From 2024-2025, Jones put up a combined -1.3 fWAR. In 2026, he’s languished in Cleveland and Chicago’s AAA systems, struggling even more since his trade to Chicago in early June, with a wRC+ of 83.
At this point in his career, it’s tough to say whether Jones qualifies as a AAAA bat. His strikeout problems have continued, with a 25% K-rate this season in AAA, a slight improvement over his 30% MLB career K-rate.
Jones seems to be the answer to the question, “What if Luke Raley was bad?” Now, with Raley hurt, there’s a chance that Jones comes up as the 26th man at some point this season. However, there are currently better options in Tacoma, believe it or not.
Boston Smith – C/OF, Double-A
Alright, now we’re talking.
Boston Smith has been the talk of the town in prospect circles this year. He’s moved from Low-A to High-A to Double-A, improving his wRC+ at each stop. He ended his Birmingham Barons career running a 196 wRC+. His Three True Outcome percentage (percent of plate appearances resulting in a home run, walk, or strikeout) is 55% in his short minor league career. He’s hit 22 home runs so far this season. He has the 8th-highest wRC+ in all of MILB.
Interested? I sure am. The Harry Ford comps are out there for a reason, but in a perfect world, Tyler Soderstrom might be more fitting as a hope for Smith. Smith’s got more pop than Ford and is a little less athletic, more likely to find himself in left field than the center field dreams that many had for Ford. Arkansas is, somehow, becoming even more interesting to watch. If Jones was a quasi-depth replacement for Raley, Smith is a longer term potential solution at an outfield corner with the valuable versatility to be a backup backstop that Seattle’s been willing to utilize frequently in recent years.
Smith slides in at 14th on the MLB Pipeline prospect rankings for Seattle immediately post-trade. Were I to re-rank the system right now, he probably slides in the back-half of the top 10, probably at 8th or 9th. He’s going to be really fun to keep an eye on, and will likely become a darling for the prospect fans. I also, sadly, would not be surprised to see him on the move again by this time next year, as he’s going to be a hot chip that others have their eyes on.
The Money
The White Sox are picking up the remainder of Castillo’s salary, which includes part of the $22.75 million owed in 2026 and all of $22.75 million for 2027. Dominguez has about $12 million left to be paid this season and next. All told, that’s $18 million guaranteed that the Mariners are saving.
I expected that in order to be able to move Castillo, the Mariners would have to either eat his salary or sacrifice getting any interesting pieces back. Saving this much cash is an absolute game-changer, giving the Mariners a good bit more flexibility to spend this offseason, given the budget their ownership chooses to operate with. That alone is why I say that even if the players in this trade underperform, this trade is still likely a win for the Mariners.
All told, this trade is a savvy move from a front office that indicates that they haven’t given up on this season entirely yet. We’ll see over the next 24 hours if Seattle is committing to this season, perhaps trading another starting pitcher to add a right-handed bat to make room for Kade Anderson, or letting things ride with a roster largely intact.
