The Mariners are headed toward the 2026 trade deadline with a clear shopping list: a proven relief arm and/or a right-handed bat. And if Seattle wants to land one of those pieces by August 3, the front office may have to put real talent in play.
That means looking hard at both the big-league roster and the upper levels of the farm system. Some of the names here are obvious. Others are the kind of prospects other clubs would circle immediately if the Mariners decide to get aggressive.
On the major league side, Luis Castillo stands out as the clearest trade chip. He’s in the fourth year of a five-year, $108 million deal that includes a 2028 vesting option, and his $24.15 million salary is a heavy lift for a mid-market club.
Seattle also has a starting pitching surplus, and the club would likely prefer to move Kade Anderson into Castillo’s spot sooner rather than later. Castillo’s 2026 season has been a little awkward, but he has kept his value intact by grinding through it, and his 3.67 ERA over his last 11 appearances still makes him appealing to a pitching-needy team.
Emerson Hancock is another arm the Mariners are willing to discuss. According to Ken Rosenthal and Will Sammon of The Athletic, Seattle has told teams it is open to moving him.
That fits the idea of Hancock as the next man up if Castillo is harder to move than expected. If Hancock goes, the return should fit a clear need, and right-handed bats are the obvious target.
He’s in the same general tier as Casey Schmitt, Matt Shaw and Spencer Steer, players who can move around the field and handle lefties.
George Kirby is a different kind of possibility altogether. If Seattle has to part with one of its homegrown aces, he belongs at the top of the list.
He checks every box: All-Star-caliber, controllable beyond 2026, and probably the toughest of the group to extend long term. But this is firmly a break-glass scenario.
Kirby should only be moved if the Mariners are getting back someone on the level of Tarik Skubal, Mason Miller or, by some miracle, Byron Buxton. Otherwise, he stays put.
Luke Raley also sits in the mix because of the roster overlap Seattle has in the outfield. He and Dominic Canzone are both left-handed hitters with defensive limitations, and one of them can be moved if it helps open a path to an everyday right-handed bat.
Canzone has more trade value because of his 137 OPS+ and club control through 2029, which is exactly why the Mariners should probably keep him. Raley may not bring much on his own, but another team could view him as a useful add-on because of his left-handed power.
The farm system offers even more possibilities, and the Mariners may have to resist the urge to treat every top prospect as untouchable.
Lazaro Montes, ranked No. 3 in the system, has rebounded after a slow start at Double-A Arkansas. A lot of that early dip was tied to the way Dickey-Stephens Park suppressed his power, and he’s since gotten back on track.
He now has 59 total home runs over the last two seasons. Still, evaluators are split on him because his profile leans so heavily on power.
That kind of projection can be volatile, which is part of why his trade value may never be higher than it is right now.
Felnin Celesten, the system’s No. 4 prospect, has been the breakout story of Seattle’s 2026 farm season. He has been outstanding, hitting .313/.407/.528 in 83 games while playing excellent defense at shortstop.
He also moved from High-A to Double-A in fewer games than Colt Emerson. Even so, the depth chart matters here.
Celesten sees Emerson at his position, and he also carried some uncertainty last year after health issues and a season in which his bat never really got going. That makes him a logical sell-high candidate, especially if the Mariners don’t want to shift him off shortstop.
Michael Arroyo, ranked No. 5, is also surging. Since his promotion to Triple-A, he’s batting .358, and since June 1 he’s hit .346/.420/.564 with a 158 wRC+ in 38 games.
His career minor league OBP is still above .400 in his fifth season. The catch is that Arroyo is almost the mirror image of Montes: hit-over-power instead of power-over-hit.
He’s a better athlete than Montes, but he isn’t a sure thing defensively in left field or at second base. That leaves the same basic question hanging over him - whether this is the peak of his value.
Jonny Farmelo, the No. 6 prospect, has forced his way into the conversation too. He was drafted seven picks after Emerson in 2023, then spent much of 2024 and 2025 sidelined by injuries.
In 2026, though, he’s pushed forward quickly, with a scorching June earning him a jump from High-A to Double-A. Even so, most of his best work has come in that one hot month, and the injury concerns are impossible to ignore for a 21-year-old.
If Seattle is looking for a time to sell high, this might be it.
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