The Seattle Mariners have made it clear they’re not playing games when it comes to addressing their first base situation. Josh Naylor finished the 2025 season as their starter, and the organization envisions him holding that role in 2026 and for the foreseeable future. Their plan is straightforward — Naylor is their guy.
However, baseball’s offseason always brings unpredictable developments, and it’s easy to see a scenario where circumstances could force Seattle to revisit an earlier consideration — Boston Red Sox first baseman Triston Casas.
The Mariners were already close to pursuing Casas last offseason when the Red Sox inquired about a trade centered around starting pitching. Boston reportedly sought one of Seattle’s young arms, Bryce Miller or Bryan Woo, in exchange for Casas. Seattle’s front office declined that proposal without much hesitation, preferring to hold onto their promising pitching depth. At the time, that decision appeared to be the right one.
The 2025 season offered proof of that judgment. While Bryce Miller struggled and ultimately lost the year to inconsistency and injury, Bryan Woo emerged as an All-Star and a foundational piece of the Mariners’ rotation. Meanwhile, Casas’ season was derailed early when he suffered a torn patellar tendon in his left knee in May, ending his campaign prematurely.
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With those developments, Seattle looked wise for not pulling the trigger on a risky deal. Still, should Naylor’s situation change — whether due to injury, performance, or financial constraints — the idea of acquiring Casas could resurface. The Mariners’ interest in strengthening their offense remains ongoing, and Casas’ raw power and upside could make him a target once again if circumstances align differently this offseason.
Mariners could get another crack at Triston Casas if the Red Sox make him available
Rumors began circulating in July that the Boston Red Sox were growing skeptical about Triston Casas’s ability to reestablish himself as a major offensive presence in their lineup. Once seen as a cornerstone piece of Boston’s future, Casas’s standing with the organization has become increasingly uncertain.
The first baseman has posted a series of cryptic messages on Instagram, which only fueled speculation about his frustration and possible departure. Meanwhile, USA Today’s Bob Nightengale reported that several rival general managers believe Boston could be an ideal landing spot for New York Mets slugger Pete Alonso, whose name has frequently surfaced in trade chatter.
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The connections between these developments are obvious. If the Red Sox were to land Alonso, that move would almost certainly make Casas expendable. Even if Boston still values his long-term upside, Alonso’s arrival would push Casas out of his natural position at first base, turning him into a logical trade chip. And if the Seattle Mariners, who have been strongly linked to Josh Naylor, were to lose out on him — which is unlikely but still possible if his price tag exceeds $100 million — that could open another line of communication between Boston and Seattle.
This potential scenario differs from previous trade discussions involving Casas. Whereas he was once viewed as a premium, buy-high target, he would now represent more of a buy-low opportunity for interested clubs. Casas is only 25 years old and remains under team control through the 2028 season, which gives him significant long-term appeal. However, his 2025 campaign was derailed by a knee injury, and before landing on the injured list, he was hitting a disappointing .182/.277/.303. The injury compounded an existing narrative about his durability issues, as he has struggled to stay healthy for extended stretches since reaching the majors.
Despite these setbacks, the Mariners could view Casas as a worthwhile reclamation project. When healthy, his offensive potential is undeniable. Between the 2023 and 2024 seasons, he posted a strong 127 OPS+, with a 162-game pace equivalent to 31 home runs.
During his breakout 2023 campaign, Casas ranked 21st among all major league hitters with a .370 expected weighted on-base average (xwOBA), a metric that reflects his ability to make quality contact. His game does include a fair amount of swing-and-miss, but he offsets that with solid plate discipline and legitimate power to all fields — qualities that have historically appealed to Seattle’s front office.
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From Boston’s perspective, any trade involving Casas would almost certainly have to bring back a pitcher, particularly one capable of stabilizing a rotation that has been inconsistent for several seasons. The Mariners, known for their deep pitching staff, could meet that demand, but not at the same high cost it might have taken last winter. If both clubs were motivated to make a mutually beneficial, buy-low trade, one theoretical framework could look something like this:
Seattle Mariners receive:
- 1B Triston Casas
- RHP Jordan Hicks
Boston Red Sox receive:
- RHP Luis Castillo
At first glance, the deal might appear unconventional, but it carries a certain logic for both sides. The Red Sox were connected to Castillo during last offseason, though trade talks reportedly fell apart due to the right-hander’s desire to restructure his contract. Now that his no-trade clause has expired, Castillo would no longer have veto power over potential destinations, removing one of the previous obstacles.
Financially, the swap would also balance out in a practical way. Castillo is under contract for $24.15 million annually through 2027, while Hicks is owed $12.5 million per year through 2026. The Mariners, by making this move, would save money in the short term and also add a dynamic bullpen arm with electric stuff — Hicks has one of the most dominant fastballs in the league when healthy. In addition, Seattle would acquire Casas, who could slot in as their long-term first baseman if he rediscovers his form from 2023.
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For Boston, the trade would remove two lingering question marks — an oft-injured young hitter and an inconsistent reliever — and replace them with near-term stability in their starting rotation. Castillo, a proven frontline starter, would immediately join Garrett Crochet to form a solid one-two punch atop the staff. His track record of durability and strike-throwing would address one of the Red Sox’s biggest issues over the past few seasons.
From a broader perspective, such a deal fits within the distinct needs and risk tolerances of both franchises. The Red Sox have been attempting to accelerate their return to contention in the American League East and need rotation certainty more than long-term potential. Meanwhile, the Mariners continue to emphasize cost control and roster flexibility. With several of their key pitchers locked into medium-to-long-term deals, they could afford to part with one high-priced veteran if it meant adding two players who could contribute for years to come.
Still, this remains speculative at best — the kind of trade scenario more at home in a fantasy baseball discussion than an actual front-office negotiation. Everything hinges on how other dominoes fall, particularly with Seattle’s pursuit of Naylor.
The Mariners’ front office appears committed to landing Naylor as their primary first-base target, and most around the league expect them to succeed in that effort. However, if negotiations stall or his market inflates beyond what Seattle is willing to pay, they may need to pivot quickly. In that event, Casas could become one of the most realistic and attainable alternatives available.
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There are, of course, risks attached to such a move. For Seattle, betting on Casas means trusting their player development staff to help him stay healthy and rediscover the plate approach that once made him one of baseball’s top hitting prospects.
His swing has proven prone to mechanical breakdowns when he presses for power, and his injury history raises long-term concerns. However, his age, contract status, and offensive upside make him precisely the type of player the Mariners typically target — young, under control, and with enough talent to blossom in a new environment.
For the Red Sox, dealing Casas would be an admission that their once-promising investment has not panned out as hoped. It would also shift the franchise’s identity, moving away from homegrown player development toward more immediate, veteran-driven contention. Bringing in Castillo would make sense for a team looking to shore up its pitching staff, but it would also mean giving up on a player who, not long ago, was viewed as a future middle-of-the-order anchor.
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Ultimately, these discussions illustrate how fluid the offseason can be. A single move — like the potential signing of Pete Alonso — could trigger a cascade of follow-up decisions across multiple teams. If Alonso ends up in Boston, Casas will likely find himself on the trading block. And if the Mariners’ preferred options at first base fall through, his name could quickly rise to the top of their list.
For now, this scenario exists purely in the realm of speculation, an intriguing “what if” for fans and analysts alike. But the underlying logic is sound: the Red Sox want certainty in their rotation, and the Mariners might soon need a first baseman with upside.
Casas could be the bridge that links those two goals, transforming a fading asset in Boston into a potential cornerstone in Seattle — if the timing and circumstances align. Until then, the Mariners’ focus remains firmly on securing Josh Naylor, a move that both the organization and most of the league still expect to happen. But if they’re forced to look elsewhere, Triston Casas could easily be waiting in the wings as their next big swing.
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