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Seattle Mariners

Mariners spring training preview: How good can the M’s rotation be in 2024?

Seattle Mariners ace Luis Castillo returns after starting 33 games a year ago.  (Dean Rutz/Seattle Times)
By Adam Jude Seattle Times

SEATTLE – If the Seattle Mariners are going to contend in the American League West in 2024, they’ll do so on the strength of their starting pitching.

Through all the upheaval and uncertainty this winter, the Mariners approach the start of spring training next week having accomplished one of their primary objectives of the offseason: They made five notable trades to reshape their lineup and augment their bullpen without having to give up a prized starting pitcher.

“And that’s really important to us,” general manager Justin Hollander said recently.

For good reason.

Boasting one of MLB’s best pitching rotations in 2023, the Mariners return the starting staff intact, and it’s reasonable to expect it could be even better this year.

Warm up those pitch clocks, folks. Pitchers and catchers are scheduled to report to the Mariners’ spring training complex in Peoria, Arizona, on Wednesday, and today we begin a preview series looking ahead to the 2024 season with a primer on the Mariners’ starting pitching. Let’s get to it.

Who’s back

All five starters who finished last season return for a pitching staff that led the AL with a 3.74 earned-run average.

Here’s how those five fared in 2023:

Luis Castillo: 33 starts, 197 IP, 3.34 ERA, 219 K, 56 BB

Logan Gilbert: 32 starts, 190.2 IP, 3.73 ERA, 189 K, 36 BB

George Kirby: 31 starts, 190.2 IP, 3.35 ERA, 172 K, 19 BB

Bryce Miller: 25 starts, 131.1 IP, 4.32 ERA, 119 K, 26 BB

Bryan Woo: 18 starts, 87.2 IP, 4.21 ERA, 93 K, 31 BB

With Castillo, Gilbert and Kirby, the Mariners have a front line as solid as any in the majors. All three ranked among the top 11 starters in the AL in FanGraphs’ Wins Above Replacement measure.

Castillo had a fine season in his first full year as Seattle’s ace, throwing a career high in innings and finishing fifth in the AL Cy Young voting. But he faded down the stretch (4.96 ERA in September) as the Mariners ultimately fell out of playoff position on the final weekend.

Gilbert and Kirby each topped 190 innings for the first time in their careers, and there’s still room for growth for both as they enter their age-27 and age-26 seasons, respectively.

Miller and Woo, called up from Double-A to stabilize a rotation that lost veteran left-handers Robbie Ray and Marco Gonzales to injuries, were as good as anyone could have reasonably expected given the circumstances. Their upside from the back of the rotation offers as much optimism as anything about the Mariners’ outlook in 2024.

Trending up

Kirby made his first All-Star team and finished eighth in the AL Cy Young voting, and there are those in the organization who feel strongly his best days are still in front of him. Kirby’s walk rate is historically good, but his strikeout rate dropped in 2023 (down to 22.7%, below league average). If he gets back to where his strikeout rate was as a rookie in 2022 (24.5%), he has a chance to break through as a bona fide ace this year.

Trending down

Mariners starters logged more innings (901) than any other rotation in MLB in 2023. That durability is a good thing, certainly. But it does invite an obvious question: Will that workload have any lingering consequences in 2024? We saw some regression from the rotation in September. At times, the starters looked worn down. At times, the two rookies’ arms just looked overmatched against the Rangers and Astros’ vaunted lineups. The Mariners had a plan to shift to a six-man rotation in mid-August, but that was quickly scrapped when rookie right-hander Emerson Hancock went on the injured list with a Grade 1 shoulder strain.

New faces

Ray and Gonzales were shipped out in what were effectively salary-dump trades. The Mariners signed 31-year-old right-hander Austin Voth, the University of Washington and Kentwood (Washington) High product, to a one-year deal last month. Voth and Trent Thornton are slated to serve in swing roles out of the bullpen, as potential spot starters.

Hancock, 24, the club’s top pitching prospect, enters spring as the No. 6 starter. He made his MLB debut on Aug. 9, allowing one run over five innings in a victory over San Diego at T-Mobile Park. He left his third start, at Houston, on Aug. 20 because of the shoulder strain and was shut down for the rest of the season. He could be in line for 10-15 starts this season as the Mariners closely monitor Miller and Woo’s innings.

Breakout potential

In 18 starts as a rookie, Woo had one of the majors’ most effective fastballs. He threw his four-seam fastball 46.9% of the time, and his two-seamer 25.6% of the time. He allowed a .207 batting average with fastballs.

Based on Woo’s pitching profile, the pitcher most similar to him last year, according to Baseball Savant, was Phillies ace Zack Wheeler. Elite company, indeed. Like Miller, Woo will need to further develop his secondary pitches to take that next step.

By the numbers

64.5: Last season, the Mariners pitching staff set an MLB record in first-pitch strike percentage at 64.5, breaking the record of 64.2 held by the 2018 Los Angeles Dodgers.

18: Mariners pitchers posted 18 shutouts in 2023, the most in the majors and a franchise record. The Mariners also set a franchise record with 1,459 strikeouts in 901 innings pitched.

0.90: Last season, George Kirby became the first pitcher in Mariners history to lead the majors in walk rate (0.90 per nine innings) and strikeout-to-walk ratio (9.05). Kirby’s walk rate was the best of any MLB starter over a full season since 2014.

14.7%: Luis Castillo led the American League with a 14.7% swinging strike rate in 2023. He ranks third in the AL in strikeouts (296) since making his debut with the Mariners on Aug. 3, 2022.

2.9: Bryce Miller generated 2.9 inches more “rise” with his four-seam fastball than the major-league average with that pitch, which ranked No. 3 among all MLB starters in 2023.