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The hottest pitching prospect in baseball joined the coldest pitching staff in baseball over the weekend.

Mariners left-hander Kade Anderson was solid in his major league debut on Saturday. The Cubs weren’t the softest landing, entering the day as the best lineup in the majors with a 111 wRC+. But the top prospect kept the Chicago batters off balance for five innings, striking out five with minimal resistance. Manager Dan Wilson left Anderson in to face the top of the order for a third time to begin the sixth — a controversial decision, but perhaps a necessary one given Seattle’s pitching implosion over the last two months — and he was quickly slugged out of the game. It was a sour note to end an otherwise strong debut, although dogpiling with his new teammates following a walk-off home run by Randy Arozarena certainly improved the mood.

The Mariners took Anderson third overall out of LSU last year. He got his first taste of pro ball this spring and blew through the competition in Double-A, posting a 1.06 ERA, a 1.78 FIP, a 40.5% strikeout rate, and a 3.9% walk rate over 93 1/3 innings. He made it as high as third overall on our Top 100 Prospects list, definitively the best pitching prospect in baseball at the time of his debut. He might have been called up sooner, but, well, I’ll get to Seattle’s pitching mess soon enough.

A spindly lefty, Anderson doesn’t stand out in his frame or velocity. He instead excels with tremendous deception. He has an unusually high arm slot for a lefty, great extension down the mound, and a whipping arm action that hides the ball well. (Kate Preusser at Lookout Landing captured a video from the umpire’s perspective in spring training, if you want to see this in action). Anderson also has a broad, four-pitch arsenal that he mixes well, with nice separation in movement and velocity to help attack the zone. Brendan Gawlowski wrote in our Mariners Top 25 Prospects list that Anderson’s “stuff plays like a power pitcher’s despite the velocity.”

We got a nice look at this arsenal on his first career strikeout. He fell behind 2-0 to Alex Bregman before getting a chase and whiff on a slider below the zone. He then dotted a fastball at the top (foul), dotted a slider at the bottom (foul), dotted a fastball away (foul), expanded the zone with a changeup (foul), and finally expanded the zone even more with a changeup (whiff) to set down Bregman.

Anderson finished his first pass through the lineup with four strikeouts, a walk, no hits, and just one hard hit ball. Now, he got a bit lucky, as the hard-hit ball was a home run that Arozarena pulled back from over the fence. (It was a big day for Arozarena.) But Anderson kept chugging on the second pass, working around a leadoff single in the fourth, and a pair of baserunners in the fifth. He threw strikes, missed bats, and got through five innings at just 72 pitches — eight pitches shy of his 80-pitch max so far as a professional. It was about as good a debut as one could expect against a lineup like the Cubs have.

Then he came back out for the sixth, which is where his debut begins to merge with the reality of the 2026 Mariners. Seattle has six starting pitchers on its active roster, and the five not named Anderson are slumping hard over the last two months. The Mariners also have a shallow bullpen made even shallower having given up one of their 13 allotted pitcher roster slots to that sixth starter. Altogether, this has been the worst pitching staff in the majors by WAR since the All-Star break. Where most teams would have called it a day after five strong innings, a pitch count approaching its limit, and a great lineup turning over for the third time, the Mariners simply don’t have that luxury. Wilson confirmed this after the game, noting Anderson “still had some pitches to go” when he sent him back out for the sixth. Maybe it would have felt nice to see Anderson end his debut on a better note. But welcome to the 2026 Mariners, kid, there are no nice feelings here.

So with the Mariners leading 2-1, Anderson returned to the mound for the sixth to face the top of the lineup for the third time. He hung a slider to Pete Crow-Armstrong, who laced a double. He got Seiya Suzuki to hit a soft grounder for an out, and lucked into a double play on some boneheaded baserunning from Crow-Armstrong. That brought Bregman to the plate. Anderson’s 80th pitch of the evening was a hanging changeup that Bregman crushed for a game-tying homer. His 81st pitch was a hanging slider that Michael Busch crushed for a go-ahead homer. There was no 82nd pitch. Anderson’s night was done.

‘Save Our Ship’

Anderson joins a Mariners rotation in a bit of an identity crisis. It’s been a bizarre season, with extreme highs and extreme lows. The unit reached the All-Star break as the top rotation in the majors by WAR, but it has been the worst rotation by WAR ever since. While an intermittent offense continues to steal the headlines for the disappointing season in Seattle, it’s the pitching that’s sunk the team below even mediocrity the last two months.

Seattle’s first rotation of the year included the main trio of Bryan Woo, Logan Gilbert and George Kirby, each with track records as 4-WAR pitchers, though each with recent injuries and inconsistencies to limit his outlook in 2026. Luis Castillo tagged along as a veteran fighting against the aging curve. And Emerson Hancock was still around in a sort of last-chance role to fill in for the injured Bryce Miller.

That’s where the Mariners ran into their first “issue” of the season: Hancock was surprisingly effective in his first few starts. He’d been sub-replacement level in 37 outings from 2023 through 2025, and was something of an afterthought entering 2026. But as Michael Rosen wrote in April, Hancock made a rare overhaul to his delivery over the offseason. The result was sudden competence and occasional dominance, highlighted by a 14-strikeout performance in May. The Mariners found themselves with another good starter.

That made things tricky when Miller was set to return from the IL. After a spectacular finish to 2024, he’d missed most of 2025 with bone spurs in his elbow, struggling to maintain his velocity between trips to the IL. He declined surgery to remove the spurs, and it wasn’t clear how he might hold up in 2026, especially as an oblique injury cut short his spring training. With Hancock pitching well, Miller was no longer an obvious upgrade, and a direct swap was out of the question. The Mariners dawdled a bit before tentatively activating Miller in May for a trial run as a sixth starter.

It just so happened that Miller arrived with his best-ever stuff. His velocity was up, his secondaries were sharp and biting, and he looked fully in control of the strike zone. When he’s on, Miller has proven tremendously talented, with a great fastball, a broad arsenal, and strong command — a sort of amalgamation of his teammate’s best qualities. It seemed he was “on” early in 2026.

That made the situation even more difficult for the Mariners. They had six true starting pitchers, with five actively pitching well, and Castillo holding his own as the type of beloved, and well-paid, veteran that can’t be cut. The solution? The team decided to stack, or “piggyback,” the outings for Miller and Castillo, helping the former ease into a full workload and limiting exposure for the latter. The plan wasn’t popular among the piglets or an anxious fanbase, but the results were fantastic. In the first three piggyback outings, Miller and Castillo covered 27 innings, allowed five runs on 12 hits, and struck out 30.

It might have worked too well. Miller was so dominant in these abbreviated outings that the team could no longer justify the constraints. The Mariners switched from the piggyback to a more traditional six-man setup in June. The group continued on, unfazed initially, dominating for a few more turns through the rotation. Seattle starters finished the first half with an 87 FIP- and the most innings pitched of any rotation, giving them 11.0 WAR — best in the majors. This was also about the time it’d become clear that Anderson would force his way to the majors at some point in 2026. It was an embarrassment of riches.

It’s been closer to just an embarrassment ever since. I don’t say that just for the symmetrical syntax. The Mariners have the worst pitching staff in the majors since the All-Star break by WAR, which was true even before last week’s dual slaughterings: 22-0 at the hands of the Brewers on Tuesday, and 19-2 at the hands Cubs on Sunday. It’s the worst the Mariners have pitched this decade.

The six-man rotation seemed to wear on the full staff over time. The extra starter removed an arm from the bullpen, without the benefit of a natural rest day offered by the piggyback. And because the bullpen lacked quality depth, without the types of mid-relief options who could reliably bridge to Andrés Muñoz late, the rotation was often asked to work deeper into games. This was especially tough as the offense slumped, creating tons of close games and high-leverage, high-stress pitches on the third pass through a lineup. This resulted a sort of snowball effect, where an early hook one night asked the bullpen to cover too many innings, which forced the next day’s starter to work too many innings to cover for the shallow bullpen, and on and on. A slump for one pitcher slowly turned into a slump for all.

Seattle shifted back to a standard five-man rotation at the trade deadline by shipping Castillo to the White Sox. There was a bit of relief with an extra arm in the bullpen. But by then, the starters were already gassed and struggling to recover on normal, shorter schedules. Miller especially has labored to maintain consistent velocity back in a full-starter role; both blowouts last week followed outings where he was unable to get through five innings, leaving a mess for those behind him. Woo is dealing with shoulder soreness and exited his last start early (though seems to have avoided the IL). The rotation continues to throw tons of innings despite its slump, as the quality of the bullpen has only gotten worse. It’s really hard to see how this corrects itself in the final six weeks. If it doesn’t, the once-vaunted Mariners pitching staff will finish below average for a second consecutive year.

Enter Anderson. It’s not clear what structure the rotation will take now that he’s here, but the Mariners again find themselves with six starting pitchers on the active roster at the time of publishing. Anderson can’t (or hasn’t) thrown more than 81 pitches in a games as a professional, and he’s injected into a rotation that’s truly desperate for the kind of innings total he likely can’t provide this season. Maybe there’s an IL stint coming for Woo or Miller. Maybe there’s a future piggyback announcement. I’m not sure. But not only are the Mariners no longer too good for Anderson, it’s fair to wonder if they might sink him to their level, as they did on Saturday.

I won’t leave you on such a downer, not for a prospect debut. Anderson is the most anticipated pitching prospect in Seattle since Félix Hernández in 2005, or at least Taijuan Walker in 2013. He looks set to make good on those expectations:

Dan Syzmborski told me ZiPS thinks Anderson is the best pitching prospect in the minors this year, and it’s not particularly close. He slots into the rotation in 2027 as a legitimately competitive arm. It’s a group that doesn’t have an ace-level pitcher but remains very deep, with six true major league-quality starters. The Mariners also have Ryan Sloan, who ranks among the top 10 prospects in baseball and is also carving up batters in Double-A; Sloan could make his debut later this year as well. Maybe it’s one of them who develops into that true no. 1 starter to help carry the Mariners in times of crisis. They need that now more than ever.