We’ve already covered the performance of the Mariners’ offense as it entered the Memorial Day weekend. Now, let’s shift our focus to the backbone of this team - it’s starting rotation.
Top of the heap
The Mariners have arguably one of the best starting staffs in baseball. It’s breathing down the neck of Philadelphia’s rotation for most quality starts. Not only that, Seattle starters have been exceptional at keeping hitters off the base paths.
One metric, which is not as good as I had anticipated, is the rotation’s .311 xwOBA. It’s better than the league average, but not by a lot. This is an unexpected development from such a talented group of arms.
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. The MLB average xwOBA = .319
That said, the individual xwOBA of several Mariners remains very good in the eyes of this nerd. Among 175 starters facing at least 150 batters, three have a top-50 xwOBA: Luis Castillo, George Kirby, and Logan Gilbert.
xwOBA of Opening Day M’s Rotation
Logan Gilbert - .288 (24th in MLB)
George Kirby - .305 (44th)
Luis Castillo - .308 (50th)
Bryce Miller - .325 (72nd)
Emerson Hancock - .378 (DNQ)
MLB average xwOBA = .318
Although rookie Emerson Hancock lacked the prerequisite plate appearances to be ranked, his .378 xwOBA does stand out from the rest in an unflattering way. Still, Hancock deserves credit for admirably filling in for Bryan Woo, who began the season on the IL with elbow inflammation.
Hancock took some punches during several of his seven outings. But he also delivered three consecutive quality starts. Solid for a rookie holding down the number-five spot in a big-league starting rotation.
As for Woo, he has an impressive .225 xwOBA through his first three starts. The Oakland, California native continued to build upon an excellent 2023 rookie campaign. Since making his MLB debut with the Mariners last June 3, the right-hander is top-25 in multiple categories among pitchers facing 400-plus hitters during this span.
Woo Since June 3, 2023
35.3 Hard-hit% (13th best in MLB)
5.4 Barrel% (4th)
25.4 K% (32nd)
7.7 BB% (73rd)
1.10 WHIP (16th)
.211 AVG (13th)
.285 OBP (22nd)
.358 SLG (22nd)
.282 wOBA (18th)
.281 xwOBA (12th)
As great as Woo has been since joining the Mariners, his kryptonite last season was the left-handed batter. During his rookie campaign, the Cal Poly product held right-handed hitters to an outstanding .268 SLG. But opponents from the left side produced an unsightly .539 SLG. So far, things are looking much better for Woo in 2024.
Still, we should temper our enthusiasm until we see a larger sample size from Woo. To date, he’s faced 33 left-handed bats. As we’re about to see with one of his rotation-mates, early success against opposite-handed opponents can be fleeting.
Another bright spot for Woo; he hasn’t allowed a home run to any of the 55 batters he’s faced this season. Unfortunately, several other Seattle starters haven’t been as fortunate in this category.
Trouble with the long ball
The 3.5-percent home rate of George Kirby is 19th-highest among qualified starters. Last year, Kirby had an average-ish 2.9-percent rate. Meanwhile, Luis Castillo (3.3-percent) is 25th-highest. It’s worth noting Castillo was plagued by home run troubles in 2023 when he allowed a career-high 28 dingers.
The Mariner struggling most is Bryce Miller (4.8-percent), who has the second-highest home run rate among qualified pitchers. The Texan has experienced four multi-HR games, which ties him with Reid Detmers of the Angels and Tyler Alexander of the Rays for second most. Cleveland’s Logan Allen leads the majors with five. It’s worth noting both Kirby and Castillo have three.
Susceptibility to the home run leads us to consider what might be driving Miller’s worse-than-average .325 xwOBA. His 24.3-percent strikeout rate is 31st-best among qualified pitchers, while the walk rate is better than the MLB average. But loud, quality contact has hurt the 25-year-old. Miller’s 13.3-percent barrel rate ranks 133rd among 134 pitchers facing 150-plus hitters this year.
Most of Miller’s issues can be traced back to difficulties against left-handed hitters this month. In the offseason, improving against lefty bats was a point of emphasis for the right-hander. His work over the winter appeared to have paid off during the first month of the season. Not so much in May.
Perhaps this month’s numbers represent randomness or the league adjusting to Miller. It’s also important to remember separating the first two months of a season does create a pair of small samples. Still, being able to succeed against left-handed bats is important to the development of Seattle’s fourth-round pick in 2021 - and Woo.
Miller isn’t the only Mariner to take a step back in May. This becomes evident when we review the monthly xwOBA of the “core four,” who’ve been in the rotation since Opening Day. Remember, xwOBA essentially represents the quality of contact allowed by a pitcher without the influence of defense. Therefore, it’s a nice way to gauge effectiveness.
In the first month of the season, all four Mariner starters had a better xwOBA than the average starting pitcher. That hasn’t been the case in May with the exception of Gilbert. The others have been worse-than-average by a large margin. Does this mean there’s a problem?
Not necessarily. But it’s clear Seattle’s starting staff has encountered more difficulties lately than it did in March and April. If we’re seeing numbers from the rotation at the end of June similar to what we’ve seen this month, it’ll be time to have a conversation. For now, I remain confident in the starting pitcher stable built by the Mariners.
Keeping the crew intact, physically
The Mariners’ rotation is teeming with young, dynamic starters. Four are 26-or-younger with Gilbert and Castillo being the oldest arms on the staff. While a youth movement is fun and exciting, there’s risk to be managed.
Ages of M’s Starters
Bryan Woo (24)
Emerson Hancock (25)
Bryce Miller (25)
George Kirby (26)
Logan Gilbert (27)
Luis Castillo (31)
Not long ago, we talked about Woo and Hancock having experienced arm-related injuries in recent seasons. Therefore, it’s reasonable to expect the Mariners to take a guarded approach with the youngsters.
Still, it’s not just the kids who merit close monitoring. Castillo, Gilbert, and Kirby were three of 12 pitchers to log 190-plus innings last season, which is an amazing accomplishment for this current era. But will this heavy workload affect the team’s success later this year?
Hard to tell. But as we discussed in November, six of 14 pitchers logging 190-plus innings in 2022 failed to make 25 starts the following year due to injury or ineffectiveness. Another starter, 2022 NL Cy Young Award winner Sandy Alcantara, made 28 starts but underwent Tommy John surgery immediately after the season ended.
Perhaps this knowledge, and all the other stuff manager Scott Servais knows about his starters that we don’t, play into his decision to pull Gilbert or Kirby from a game when they’re cruising along with a relatively-low pitch count. I realize it’s a hot button topic on Mariners Twitter whenever Servais makes such a decision. Personally, being critical about someone else’s decision without having all of the information they have would say more about me than the decision-maker.
Stay the course
The Mariners are currently in a good place with a starting-five possessing enviable talent, plus Hancock in reserve. But as we recently discussed, the average MLB team used 14 starters last year. To date, Seattle has used six starters with the most likely backup behind Hancock being left-hander Jhonathan Díaz, who’s pitching for Class-AAA Tacoma. After that, it’s a bit murky.
Seattle’s management has resisted trading one of its MLB-ready starters and should continue to do so. While the notion of subtracting from the Mariners’ strength makes for a great talking point, doing so doesn’t make any practical sense for a team struggling to produce offensively.
At least it doesn’t to me.
My Oh My…