Why the Mariners may have one of the best rotations in baseball in 2023

Tristan Casady
8 min readMar 10, 2023

Top-end talent, durability, and depth can propel this group to the top in all the sport

Luis Castillo and his signature fist pump in the 2022 season; courtesy of TNT

The trade deadline acquisition and subsequent extension of starting pitcher Luis Castillo marked the culmination of a years-long effort to build an elite rotation in Seattle. Prior to Luis’s arrival, the M’s had built a solid foundation of homegrown talent and a free agency splurge. What it lacked was a true “Ace”- a guy who could go toe-to-toe with the best of the best when it mattered most. Castillo’s dominance in game one of the AL Wild Card solidified his status as Seattle’s first true ace since King Felix himself. Castillo now leads a starting rotation poised to take a huge leap forward in 2023 to become among the best in baseball — if not the best.

As mentioned above, the Castillo acquisition was the final chapter of an organizational overhaul of pitching development. It was a journey that started in 2017 with the trade for Gonzaga graduate Marco Gonzales from the Cardinals to anchor the rotation after years of mediocrity. The subsequent years saw the Mariners target several college arms in the early rounds of the draft: Logan Gilbert (2018), George Kirby (2019), Emerson Hancock (2020), and Bryce Miller (2021). Gilbert and Kirby have already made massive contributions to the big league team while top prospects Miller and Hancock are poised to contribute as early as this season. Throw in the very underrated signing of Chris Flexen after his success in the KBO and the Mariners quietly assembled the makings of an elite rotation. Now they just needed a couple of top-end arms to lead this young group. Dipoto finally shelled out in free agency to sign 2021 AL Cy Young winner, Robbie Ray, showing that the Mariners were ready to compete after a multi-year “retooling” process.

Jerry Dipoto and Robbie Ray at Ray’s introductory press conference; via MLB.com

Let’s break down this rotation and why it can be so dominant.

The Ace(s)

I’ll start this section with the pitcher many M’s fans were disappointed with following the conclusion of last season. Many argue Robbie Ray has not lived up to his lucrative contract but I think that has everything to do with expectations and the current market value of quality starting pitching. I don’t believe that the Seattle front office signed Ray expecting him to replicate the level of dominance that won him the Cy Young. I do believe, however, that they were willing to spend on a guy that would lead a young rotation and deliver quality innings in large volume. That is exactly what Robbie did for the Mariners in 2022 — even if the Yordan Alvarez walk-off homerun still leaves a bad taste in your mouth. Granted, Ray’s valleys were much lower than you’d like to see but don’t let that overshadow how strong of a pitcher he was for the majority of the year. Following a rough outing against Houston on June 6th, Ray finished the season with a very solid 2.97 ERA and 135 strikeouts over 118.1 innings. Ray will have his occasional blowup starts, sure, but I think it’s fair to expect a 2023 season somewhere between his 2022 and Cy Young-winning 2021. Give me 200 innings of that and we have one of the better arms in the American League.

When the Mariners swung a blockbuster trade at the deadline, their goal was to land a true number one starter for their playoff push; and boy did they ever. Luis Castillo immediately showcased video game stuff and equally impressive poise on the mound. Nicknamed “La Piedra” (The Rock) for a reason, he saved his best outings for the biggest moments. In his first home start for the M’s, he went toe-to-toe with Gerrit Cole and the Yankees and finished with 8 shutout innings, easily one of the most dominant pitching performances by a Mariner in years. That night and the rest of the season, fans got to see his 100 mph fastball with crazy movement, Felix-esque changeup, and disgusting slider all thrown with regularity. Yet, I think there’s still room to improve this season. Castillo’s slider and fastball were the best they’ve ever been in 2022 as they continue to trend up; and there should be no doubt that his changeup — which rated as only average this year — can perform as it has in prior seasons when it was one of the singular best pitches in baseball according to Statcast. If he stays healthy in 2023, the two-time All-Star should have no difficulty throwing close to 200 innings and a sub-3.00 ERA. A season like that would cement La Piedra’s status as a bonafide ace and potential top-10 starter in baseball.

The budding stars

Logan Gilbert and George Kirby have already made quite the name for themselves early in their respective careers. Gilbert — or his alter ego “Walter”- entered 2022 looking to build off of his up-and-down rookie season. He did just that, posting a 3.20 ERA and logging just over 185 innings. While the advanced numbers point to him regressing in 2023, I would argue that it instead shows he was able to produce without realizing his full potential. With elite velocity and ridiculous extension due to his 6'6 frame, Gilbert has plenty of room to refine his pitch arsenal ahead of this season. He has a top-25 fastball at his disposal but will need to find a way to rely less heavily on it after throwing it over 53% of the time last season. If Gilbert can mix in more of the curveball and — most importantly — the changeup or newly developed splitter, he can keep hitters from waiting on the fastball to do damage. As a constant pitch tinkerer, I’m confident Gilbert can make these changes, and when he does he has the potential to be one of the best 30 pitchers in baseball.

Gilbert in a shutout win over the White Sox; courtesy of MLB.com

As good as Logan Gilbert has the potential to be, George Kirby can be even better. Kirby represents a new trend in pitching development where teams opt for pitchability (command, sequencing, etc.) over raw stuff (velocity, movement, etc.) The Mariners knew they had a guy with other-worldly command and feel for his pitches when they selected him in the first round, and that has more than translated at the big league level. What took Kirby to the upper echelon of bright young arms was his ability to develop elite velocity on his fastball and improved secondary pitches. With a much more effective pitch arsenal and the sparkling command he’s always had, Kirby could have made a serious push for AL Rookie of the Year if it wasn’t for his own teammate. In his rookie year, Kirby’s 4-seamer ranked as the 9th-best fastball in all of baseball according to Statcast. Conversely, his offspeed and breaking pitches were hit fairly hard amidst his success, which I believe will only improve for a guy that constantly works to optimize his repertoire. If he can get the slider, curve, or changeup to even be average out pitches in 2023, Kirby and his 96th-percentile walk rate could legitimately finish as a top-15 starter in all of baseball.

Kirby dominating the Astros in Game 3 of the ALDS; courtesy of Steph Chambers/Getty Images

The wild card

This is where things get interesting. The final spot in the rotation is one to watch throughout 2023 and has the potential to elevate or sink the rotation as a whole. The Mariners figure to begin the season with one of Marco Gonzales or Chris Flexen in the number 5 spot — most likely Marco — while the other — most likely Flexen — slots into a relief role or is even traded for offensive help. Marco and Flex are known quantities at this point in their careers, and the Mariners would likely be okay rolling either of them out every fifth day. They’ll eat innings and keep the team in the game most nights even if it’s not exciting. Marco’s fall from staff Ace to fringe rotation arm probably has more to do with the addition of quality starters than anything else, but his diminished command can no longer overcome his inability to generate swings and misses (1st percentile in strikeouts last season!) The bulldog’s consistent contributions to the Mariners throughout this rebuild should not be understated even if that time may be coming to an end sooner rather than later. Ultimately, for a team that has world series aspirations — let alone winning the division — “okay” is simply not enough.

Marco in a rough outing against the Yankees; courtesy of MLB.com

Despite trading away multiple arms, Dipoto still has a collection of high-end pitching prospects knocking on the door of the big leagues as soon as opening day. Former first-round pick Emerson Hancock — now fully healthy — has looked strong this spring and will look to recapture the form that made him a top-10 pick in the draft. Taylor Dollard won a lot of games and amassed a lot of quality innings at AA Arkansas last season and probably could hold his own in the majors right now. However, the most exciting option to take over Marco/Flexen’s spot in the rotation is Bryce Miller. For a guy that only became a starter in his last year at Texas A&M, it’s remarkable how quickly he’s established himself as a top-tier prospect in the eyes of the Mariners and a consensus top-100 prospect in all of baseball. It’s easy to see why the team is so bullish on Miller as he rode his upper-90s fastball and nasty slider to a dominant 2022 campaign in which he rose two organizational levels (Low A to AA). If he can maintain solid command and continue to refine a tertiary pitch to complement his two elite offerings, the Mariners may have another frontline starter on their hands. He could slot into the bullpen on opening day and make an immediate impact as a high-leverage reliever, but I have no doubt that he can seize a spot in the starting rotation very soon where he should thrive immediately.

I must acknowledge that this rotation is still very young and relatively inexperienced. We may know what we’re going to get from Ray and Castillo, but for Kirby, Gilbert, and eventually Miller/Hancock, there is still some unknown. Volatility is to be expected, but everything I’ve seen from these young Mariners pitchers points to even greater things ahead. Seattle has quickly become a pitching powerhouse in terms of development and coaching, now we can reap the benefits. If the Mariners can replicate or build upon their magical 2022 season, it will be on the back of this rotation. With continued good health and expected improvement, it’s not hyperbole to say this group has the pieces to compete with the star-laden Mets, Yankees, and Astros for the very best in all of baseball.

Feel free to leave any questions or comments below!

Twitter: @tristancasady

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