At some point in the coming days, the Mariners are planning to shift to a six-man rotation, president of baseball operations Jerry Dipoto told Adam Jude of The Seattle Times. Despite season-ending injuries to Robbie Ray, Marco Gonzales, and Easton McGee, the Mariners boast one of the deepest rotations in baseball. As Dipoto put it, “Our one through six in the starting rotation … can pitch with anybody in the league.” That being said, most of Seattle’s starting pitchers are young and inexperienced; in other words, they could benefit from some extra rest.

The current starting five consists of Luis Castillo, George Kirby, Bryce Miller, Logan Gilbert, and Emerson Hancock. Bryan Woo is expected to return from the injured list and rejoin the rotation early next week (Twitter link). Of those six, only Castillo and Gilbert had pitched a full major league season before this year. Miller, Hancock, and Woo are rookies, while Kirby, still just 25 years old, is on pace to pitch his first qualified season. He threw 130 innings last year in his rookie campaign.

As Jude reports, the Mariners plan to go with a six-man rotation once Woo comes back from the IL. He could have supplanted Hancock, who has struggled in his first two big league starts (5.40 ERA, 9.8% strikeout rate), but it seems Seattle is comfortable giving the top prospect a longer leash. However, Dipoto did mention that the team might “piggyback” Woo and Hancock down the stretch after trying out a six-man rotation for a week or two.

Additional rest will be most beneficial for Seattle’s young arms, but it should help the rest of the rotation, too. Castillo has been one of the more durable starters in baseball over the last six years, making 158 starts with a 3.58 ERA. Only six pitchers have thrown more innings in that time, so he could certainly use some time off. The same goes for Gilbert, who had thrown 338 2/3 big league innings before his 26th birthday. Since making his debut in 2021, he ranks tenth in the majors in starts and 15th in innings pitched. Dipoto acknowledged that Castillo, Gilbert, and Kirby will have to shoulder plenty of responsibility should the Mariners make the playoffs, and some extra rest now should help them stay fresh for a potential postseason run.

However, while this arrangement will benefit everyone in the rotation, it will make things harder on the bullpen. If the Mariners carry six starting pitchers, they will only have room for seven relievers on the 26-man roster. Furthermore, it’s not as if Seattle’s young starters have been pitching especially deep into games. Woo and Hancock are averaging exactly five innings per start, while Miller is averaging five and a third. Castillo, Kirby, and Gilbert are averaging about six innings per start, but those three will pitch less often with a six-man rotation. Thus, the bullpen will have a heavier load to carry for as long as the six-man rotation experiment lasts.

Thankfully for Seattle, they have one of the best bullpens in baseball. Mariners relievers rank second in ERA and third in FanGraphs WAR, and they’ve still had excellent results since losing closer Paul Sewald, pitching to a 1.89 ERA since the trade deadline. Moreover, they have essentially been using a seven-man bullpen as it is. Andrés Muñoz, Matt Brash, Trent Thornton, Tayler Saucedo, Justin Topa, Isaiah Campbell, and Gabe Speier have combined to pitch 45 2/3 of the 47 2/3 innings the Mariners bullpen has thrown since August 1. The Mariners will be counting on those seven to handle a difficult workload for the next while, but eventually, they should get some relief if Woo and Hancock do, in fact, move into a piggyback arrangement.

Gonzales and Ray are expected to rejoin the club at some point in 2024, at which point the Mariners will have an embarrassment of riches in the starting rotation. Gonzales is under team control through 2025, while Ray is signed through 2026. Castillo, meanwhile, is signed through 2027 with a vesting option for 2028. Gilbert is also under team control through 2027, Kirby through 2028, and the rookies through at least 2029 (pending further optional assignments).

With so many capable starters (and so many arms that will need a 40-man spot over the winter), there’s a good chance Dipoto will look to make a trade. The Mariners were reportedly willing to consider dealing one of their starting pitchers this summer, and they’ll have a better opportunity to do so over the offseason. The Cardinals, who have a surplus of young hitters but not nearly enough pitching, could be an interesting trade partner. They will be looking to add multiple starters this winter, and they were already linked to Gilbert earlier this year.