The Mariners’ big-league season may have been a grind, but the pipeline kept humming. Even after Colt Emerson graduated from prospect status, Seattle still finished in MLB’s top 10 farm-system rankings last month, and a lot of the organization’s best talent is already knocking on the door at Double-A or beyond.
That’s the backdrop for a group of six prospects who boosted their stock in 2026, with several climbing sharply in MLB Pipeline’s rankings.
Kade Anderson headlined the bunch in a big way. The No. 3 pick in the 2025 draft entered the year as one of the sport’s top pitching prospects, then went out and put together a monster season at Double-A Arkansas.
In 18 starts for the Travelers, the 22-year-old lefty logged a 1.06 ERA, 0.64 WHIP and a 10-1 record, piling up 135 strikeouts in 93 1/3 innings. He held opponents to one or fewer runs in 12 of those outings, with nine scoreless starts mixed in.
That run pushed him to No. 5 in baseball after beginning the year at No. 21.
He was promoted to the majors on Aug. 22 and has gone 1-1 with a 4.29 ERA and 1.38 WHIP over 21 innings in his first four MLB starts, but he’ll still qualify as a prospect heading into 2027.
Ryan Sloan also took a meaningful step forward. The second-round pick from 2024 didn’t dominate Double-A the way Anderson did, but the 20-year-old right-hander still handled the Texas League well for someone 4 1/2 years younger than the average Double-A player.
Sloan posted a 3.73 ERA, 1.13 WHIP and a 4-4 record while striking out 128 and walking only 16 over 101 1/3 innings in 22 starts with Arkansas. He had three outings of at least six innings with one or fewer runs allowed, and he struck out a career-high 11 batters twice.
His rise from No. 33 to No. 8 in baseball reflects that progress, and he now ranks third among pitchers behind Anderson and Pittsburgh’s Seth Hernandez.
Michael Arroyo kept doing what he does best: hit. The 21-year-old earned a Sept. 6 promotion to the majors after a strong year across Double-A and High-A, where he hit .283/.359/.471 with 18 homers, 23 doubles and five triples in 114 games.
His walk rate dipped to 8.1% from 12.4% last year, but his strikeout rate also ticked down, from 18.7% to 16.4%. That season moved him from No. 67 to No. 44 in baseball and bumped him past good friend Lazaro Montes to No. 3 in the Mariners’ system.
Felnin Celesten was one of the year’s clearest rebound stories. After slipping in previous prospect rankings, the switch-hitting shortstop pushed his way back into the conversation with a breakout campaign across High-A and Double-A.
In 111 games, he hit .273/.363/.458 with 16 homers, 24 doubles, four triples, 25 steals, 54 walks and 120 strikeouts. The power jump stood out most: 10 more homers and 16 more extra-base hits than a season ago.
He also had a 25-game hitting streak in High-A and cleaned up his defense, dropping from 29 errors last year to 24 this year while playing 157 2/3 more innings in the field. Celesten is back in MLB Pipeline’s top 100 at No. 49 after opening the year outside it, and he moved from No. 7 to No. 4 in Seattle’s system.
Mason Peters made noise in his first pro season. The fourth-round pick in 2025 was overpowering at Single-A Inland Empire before moving up to High-A Everett.
Over 14 starts in Single-A, the 22-year-old left-hander put up a 1.43 ERA and 0.85 WHIP with 68 strikeouts and 12 walks. For the full year, Peters finished with a 2.10 ERA, 0.95 WHIP, 87 strikeouts and 19 walks across 64 1/3 innings in 18 starts.
That performance moved him from No. 15 to No. 11 in the Mariners’ farm system.
Colton Shaw made the biggest leap among players who weren’t on the initial list or drafted this year. The 23-year-old right-hander went from unranked to No. 20 in Seattle’s top 30 after a strong season at High-A Everett.
A 2025 seventh-round pick out of Yale, Shaw posted a 4.43 ERA and 1.15 WHIP with 121 strikeouts and 19 walks over 111 2/3 innings in 23 outings, 22 of them starts. He was a two-way player in his first two seasons at Yale and didn’t fully commit to pitching until 2024.
In Other News...
White Sox Just Took On A Rotation Risk Fans Can't Ignore
Luis Castillo was supposed to give the White Sox a stabilizing presence after Seattle dealt him before the Aug. 1 trade deadline, but the early returns have been rough enough to keep the move under a microscope. Chicago took on the remaining commitment with the idea that a proven right-hander could help steady the rotation, yet the results since the deal have gone the other way, creating an uneasy contrast with what he had already done in Seattle.
For Mariners fans, the trade always carried some long-term significance because Castillo still has $21.6 million owed in 2027, and the White Sox are now living with that ledger as well as the on-field uncertainty. He had posted a 5.06 ERA in 99.2 innings with Seattle before the deal, so the underlying concern was already there, and the jump to a 7.76 ERA in 29 innings since joining Chicago has only sharpened the question of how much trust this arm can command over the next stretch. [Read more 🡒]
Mariners Risk Wasting Emerson Hancocks Breakout At The Worst Time
Emerson Hancocks 2026 season has given the Mariners something they badly needed: stability in the rotation. After making 25 starts and turning in solid numbers, the right-hander looked like one of the quieter success stories on a staff that has had to manage plenty of moving parts. Then a left adductor strain on Aug. 28 sent him to the injured list, and with the season running short, Seattle no longer has the luxury of a normal starters rehab path.
That is why the conversation has shifted toward a possible relief role for Hancock the rest of the way, even as the Mariners weigh how his return fits alongside Bryce Miller and the rest of the rotation picture. Manager Dan Wilson has made clear that Hancocks health comes first, but the team still has to decide whether there is enough time for the kind of buildup a starter usually needs. For a club trying to maximize every inning in September, the choice could shape more than just this years finish. [Read more 🡒]
Mariners Suddenly Have A Real Andrs Muoz Problem In The Ninth
Andrs Muoz has still piled up enough save chances to give the Mariners confidence that the ninth inning belongs to him for now, but the year has not come nearly as smoothly as the raw totals might suggest. He has turned in a 4.25 ERA and a 1.18 WHIP across 55 innings, and while his fastball has played up and the strikeouts are there, the contact he has allowed has been louder than Seattle would like from its closer.
The result is a bullpen conversation that feels bigger than one rough stretch. Muoz has already blown 10 saves, the most in the majors, and the Mariners now have to sort through what that means for the back end of games even as his $8 million club option looks likely to be picked up. For a team trying to lean on its relief corps in the stretch run, the question is no longer whether Muoz has the stuff to close, but whether Seattle can keep treating the ninth inning as his alone. [Read more 🡒]