Bryce Miller

The Mariners are shuffling up their pitching staff. They’re set to select the contract of top pitching prospect Bryce Miller, as first reported by Joe Doyle of Future Star Series, and they’ve also passed right-hander Diego Castillo through waivers and assigned him outright to Triple-A Tacoma, per Ryan Divish of the Seattle Times (Twitter link).

With that outright, Castillo is no longer on the 40-man roster. He has the three years of service time needed to reject the assignment in favor of free agency, but he doesn’t yet have five years of service, so rejecting the assignment would mean forfeiting the remainder of this year’s $2.95MM salary. As such, he’s sure to accept an assignment to Tacoma and remain with the organization.

It’s been a swift decline for Castillo, who was one of the top relievers moved at the 2021 deadline, going from Tampa Bay to Seattle in exchange for minor league infielder Austin Shenton and righty JT Chargois. Castillo was quite effective in his first season and a half with the Mariners, pitching to a 3.42 ERA with an above-average 25.5% strikeout rate against a slightly below-average 9.4% walk rate. He held opponents to just a .196 average, yielded only 1.06 HR/9 and kept the ball on the ground at a 48.5% clip.

The 2023 season, however, has been a disaster for the 29-year-old Castillo. In 8 2/3 innings so far, he’s walked as many batters as he’s fanned (seven) and also plunked a hitter. He’s already served up homers to two of the 41 batters he’s faced after surrendering only five in 54 1/3 innings (222 batters faced) a year ago. The 94.2 mph average on his fastball, meanwhile, is down 1.6 mph from last year’s mark of 95.8. Overall, he’s sitting on a 6.23 ERA and has allowed 15 of 41 opponents (.366) to reach base.

Any other team could’ve claimed Castillo, but it’s generally rare for other clubs to claim struggling players of this nature when they have a guaranteed contract of some note. The Mariners could’ve waited to make this move but likely saw Castillo’s struggles as a way to open a 40-man roster spot to accommodate their top pitching prospect. If another club had claimed Castillo, the Mariners would’ve been spared the remaining $2.44MM of this year’s salary, but they’ll instead hope he can round back into form with the Rainiers.

Turning to Miller, he’ll make his big league debut when he takes the mound tomorrow against the A’s. He last pitched on April 25, so he’ll be plenty rested. The 24-year-old has had a rough start to his 2023 season in Double-A, pitching to a 6.41 ERA in 19 2/3 frames, but he entered the year among the top 100 prospects at Baseball America, FanGraphs and MLB.com. He currently ranks 92nd, 89th and 85th, respectively on those three lists.

The handful of shaky outings to begin the year, it seems, won’t dissuade the Mariners from hoping he can follow Logan Gilbert and George Kirby to be the next member of the team’s vaunted farm system to step into the Major League rotation. Seattle has already lost Robbie Ray for the season and has seen Chris Flexen struggle in the fifth spot of the rotation, so it stands to reason that Miller could potentially seize a permanent job, if not tomorrow then certainly at some point this season.

Despite his rocky start to the 2023 season, Miller is seen as a potential mid-rotation starter, thanks in no small part to a heater that sits upper-90s and can bump 100 mph. He pairs that with a slider and changeup, and the former college reliever has improved his command as the Mariners have stretched him back out as a starter.

Miller split the bulk of the 2022 season between High-A and Double-A, logging a combined 3.16 ERA with a 30% strikeout rate and an 8.5% walk rate in 133 2/3 innings. He allowed just 0.67 HR/9 and kept the ball on the ground at a slightly above-average rate. And, given that sizable workload in ’22, there’s little reason to think the Mariners will feel the need to monitor his innings total too aggressively this season.

If Miller were to stick in the big leagues, he’d be on pace to qualify for arbitration as a Super Two player following the 2025 season and reach free agency at the conclusion of the 2029 season. Of course, future optional assignments could alter one or both of those timetables considerably.