Mariners Primer: Starting Rotation

Seattle starters were outstanding last year. Can they do it again in 2025?

Lindsey Wasson / AP Images

Once again, the rotation was the backbone of the Mariners’ roster in 2024. The staff led MLB with 92 quality starts, finishing well ahead of the second place Phillies (80). Being tops in baseball was the norm for Seattle starters, who were number-one in multiple categories.

The Mariners needed just seven starters to get through the season - the fewest used by any club. Five pitchers absorbed the brunt of the workload: Luis Castillo, Logan Gilbert, George Kirby, Bryce Miller, and Bryan Woo. Covering the 13 contests not handled by the core five were Emerson Hancock (12) and Jhonathan Díaz (1).

My preferred metric for gauging pitchers is xwOBA. In the case of the Mariners, every member of the core five had a top-50 xwOBA.

xwOBA of M’s Starters with MLB rank*

Bryan Woo (.257) - 4th in MLB
Logan Gilbert (.276) - 12th
George Kirby (.288) - 22nd
Bryce Miller (.299) - 38th
Luis Castillo (.306) - 49th
Emerson Hancock (.368) - n/a

MLB Average for SP’s = .317

* 400-batter minimum

Hancock didn’t qualify for our ranking. He only faced 266 big-leaguers when he wasn’t starting for Class-AAA Tacoma.

Expected Weighted On-Base Average (xwOBA) uses quality of contact (exit velocity and launch angle) to determine what should’ve happened to batted balls. A key advantage to xwOBA is defense (good or bad) doesn’t influence it. This gives us a truer sense of how a hitter or pitcher is performing.

Gilbert led baseball with 208.2 innings in 2024. Going deep in games is the right-hander’s superpower. He’s one of just four pitchers with 185-plus innings in each of the last three seasons. The others were Logan Webb, Aaron Nola, and Corbin Burnes.

Last year was the best of Gilbert’s young career. He tied for second-most quality starts, had the lowest WHIP, and boasted an 11th-best strikeout rate. The baseball establishment took notice of the Stetson product’s excellence, which led to his first All-Star selection and a sixth-place finish in AL Cy Young voting.

Kirby continued his mastery of the strike zone, which is reflected in the lowest walk rate among qualified pitchers. But opponents did have a .245 AVG and .393 SLG against him, which were average-ish and higher than expected for someone with his pedigree.

Among 107 starters with 120-plus innings, Miller had the 11th-highest barrel rate. However, the Texan experienced a dramatic second-half turnaround. Following the All-Star game, he was top-five in multiple categories: ERA, AVG, OBP, WHIP, wOBA.

Perhaps in the long run Woo ends up being the true gem of Seattle’s loaded rotation. As noted earlier, his .257 xwOBA was fourth-best, tying him with 2024 AL Cy Young Award winner Tarik Skubal. Not a surprise considering the 25-year-old led the same group of pitchers in walk rate and yielded the fourth-lowest barrel rate.

Still, the challenge facing Woo and the Mariners is keeping him healthy. Last year, he eclipsed his previous career high for innings. But the Cal Poly product began the season on the IL with elbow inflammation and subsequently suffered a hamstring strain. As you can see below, injuries have affected Woo’s availability over the past four seasons.

Last year’s Opening Day starter, Castillo, had a subpar season by his standards. He had a better-than-average 3.64 ERA in 30 starts and 175.1 innings. Yet, there were several unfavorable stats on the three-time All-Star’s ledger.

For a second consecutive season, Castillo allowed 25-plus home runs. Moreover, opponents had a 41.7-percent hard-hit rate against the 32-year-old. Only 13 pitchers with 120 innings were worse than Castillo. With these numbers in mind, it's not surprising that opponents had a .412 SLG against the Dominican Republic native. The MLB average SLG was .399.

Whether this matters or not, it’s worth noting Castillo’s average four-seam fastball velocity has decreased about 2-mph since 2022. During this three-season period, his strikeout rate as a Mariner dropped from 28.9-percent to 24.3-percent last year.

With Seattle last season, Hancock had a 4.75 ERA in 60.2 innings with opponents having a .783 OPS against him. League-average was .711. Furthermore, his offerings didn’t generate a ton of swing and miss.

Only eight of 277 pitchers had a lower strikeout rate than Hancock did (14.7-percent). This aligns with what he produced with Tacoma (16.7-percent). When the 25-year-old reached two strikes on a hitter, he didn’t experience as much success as most of his peers did. Hancock’s 13.8-percent whiff rate was seventh-lowest among 406 pitchers with at least 200 two-strike counts.

Not putting hitters away led to a lot of balls in play. Unfortunately, Hancock allowed loud contact on a frequent basis. His 46-percent hard-hit rate was 10th-highest, while the former Georgia Bulldog’s 9.9-percent barrel rate was two points above the MLB average. All of this contributed to 12 home runs in 60 frames and a suboptimal .456 SLG.

Home sweet home

We already mentioned opponents having a .412 SLG against Castillo. But it’s worth noting his SLG at T-Mobile Park was a more respectable .379. On the road, the eight-year veteran had an unappealing .452 SLG.

Castillo wasn’t the only Mariner starter to experience more success at home than away from the Emerald City – they all did. If you want to understand why, check out the great piece by Mike Petriello of MLB that details the factors making Seattle’s home field baseball’s most extreme pitchers park.

To help illustrate this point in a simple way, let’s consider the home and away OPS of Seattle’s core five and Hancock.

Every Mariner benefited from pitching at T-Mobile Park. Some more than others. Hancock struggled at home but was worse on the road. Castillo was better-than-average in Seattle, although not as good as he was during the 2022 and 2023 campaigns (.571 OPS). Also, Kirby was subpar on the road.

Having said all that, I do feel a segment of the fan base and some in the media overreacted to the noticeable gap between the Mariners’ home and road numbers. I mean, the staff having a decided home field advantage shouldn’t come as a surprise to anyone if they took the time to read Petriello’s outstanding article.

Could the rotation have performed better away from Seattle last year? Sure, but consider this. The Mariners made the most quality starts on the road in 2024. How bad could they have been?

Most Road Quality Starts

SEA (40)
KCR (37)
PHI (34)
SDP (34)
HOU (33)
ATL (33)
BAL (33)
TOR (31)

Looking ahead

Once again, the Mariners will rely on their core five. However, one member won’t be available for a little while. Kirby has been inactive for a few weeks due to shoulder inflammation. The Elon alum recently resumed soft tossing but is expected to miss most of April.

The team has a contingency for this unplanned absence - Hancock. Last year, the sixth overall pick of 2020 covered the first month of the season when Woo was sidelined. Now, he’ll hold down Kirby’s spot.

Starter pool

Logan Gilbert
Luis Castillo
Bryce Miller
Bryan Woo
Emerson Hancock
George Kirby *
Jhonathan Díaz +
Blas Castano
Logan Evans +
Brandyn Garcia +
Casey Lawrence +

+ Not on 40-man roster
* Injured

If the Mariners were to need another rotation arm before Kirby returns, the team has a variety of depth options to consider. Díaz was the seventh starter used last year. But the club designated him for assignment in February to move him off the 40-man roster.

In his latest roster projection for the Seattle Times, Ryan Divish identified right-handers Casey Lawrence and Logan Evans as “next up” in the starter pipeline. Each would have to be added to the 40-man roster before appearing with the big-league club.

The 37-year-old Lawrence, who has been almost exclusively a reliever during his MLB career, appeared in 11 games with the Mariners in 2018. That said, Lawrence has started 20-plus games in the minors in each of the last three years, including 29 with Tacoma last season.

At the other end of the experience spectrum, Evans is just 23 and hasn’t pitched above AA. Perhaps the Pitt product would be the team’s preferred choice for an extended period with Lawrence getting the call for a spot start or two, as Díaz did in 2024.

Outlook

As good as the rotation was in 2024, there are several issues worth monitoring as the season gets underway.

Does Castillo rebound from one of his least effective seasons? Or, is what we saw last year the onset of age-related regression? Is the drop in velocity relevant or a nothing-burger?

Woo having forearm- and elbow-related issues in two consecutive seasons is troubling to a nerd with admittedly zero pitching experience or medical expertise. Will the Oakland, California native make 30-plus MLB starts for the first time ? Or, does the injury bug make another visit in 2025?

It’ll be interesting to see how much Hancock has grown since last season. Improving upon his shaky 2024 could be a season-changer. Especially, when it’s likely he’ll receive more opportunities to start games with the Mariners in the months after Kirby’s return from the IL.

The offense must be more productive and consistent than it was in 2024. But in the end, the overall success of the season likely hinges on the quantity and quality the rotation can provide. Personally, I like the chances of the staff delivering the goods.

What a great time to be a Mariners fan.

My Oh My…