Why the Mariners are considering a six-man rotation

This story was excerpted from Daniel Kramer’s Mariners Beat newsletter. To read the full newsletter, click here. And subscribe to get it regularly in your inbox.

KANSAS CITY -- Bryan Woo raced around the outfield grass early Thursday morning at Kauffman Stadium to get loose, and looked energetic doing so. He had good reason to be, given that he just completed a 30-pitch bullpen session that has him on track to be activated from the 15-day injured list at some point next week in Chicago.

When he is, the Mariners must subtract an arm from their roster -- and they’ll do so from their bullpen, as the club plans to install some semblance of a six-man rotation, Mariners president of baseball operations Jerry Dipoto said in an interview outside the visiting clubhouse before Julio Rodríguez’s five-hit, five-RBI performance lifted the club to an emphatic win.

“There’s going to be some combination of six starting pitchers,” Dipoto said. “It's just a matter of whether they're starting in six consecutive games or one of them is helping another get through.”

After their series against the White Sox next week, the Mariners will be off on consecutive Thursdays before a 13-game stretch with no days off. They then have each of the following two Thursdays off (Sept. 14 and 21). It’s that stretch in between that the Mariners will deploy this new tactic before reassessing, at which point their postseason pursuit will be more clear.

The calculus behind the decision has been clear for weeks, if not months. Seattle was always going to have a workload-management situation with three rookies contributing to the rotation in the wake of season-ending injuries to workhorses Robbie Ray and Marco Gonzales.

Mariners’ rookie workload in 2023, including Minors (previous career high)

Bryce Miller: 111 (133 2/3)
Woo: 99 (57)
Emerson Hancock: 108 (98 1/3)

“It wasn’t our intent to start the season. ... There are challenges that our young pitchers, [who] have performed so far beyond what could possibly have been expected of them,” Dipoto said. “Now we find ourselves in a pennant race and they need help.”

The pros

The Mariners can better govern the workloads of everyone across their remaining 41 games. Seattle’s rotation has compiled 679 2/3 total innings, the third-highest in MLB, which is a testament to the team’s greatest strength.

That said, there are many other metrics in play when the Mariners made this assessment, and aside from Luis Castillo and Logan Gilbert, their entire rotation has been on a schedule beyond four days’ rest over the past two seasons based on the Minor League schedule, which typically operates on a six- to seven-day rotation with most teams playing a week-long series against the same opponent.

“It was a push last year to get ready on five,” Miller said. “There's also kind of just the uncomfortable feeling of like you throw the same team Tuesday and Sunday.”

Woo has established himself as not just a fill-in, but a legitimate reinforcement.

“Obviously, the innings and the workload, but also just like going to the big leagues and stuff is an adjustment,” Woo said. “The travel, more stressful innings, five-man rotations, it's just a lot of different factors.”

The Astros deployed a six-man rotation when Lance McCullers Jr. returned from the IL last year on Aug. 13 and saw their rotation compile an MLB-high 10.6 wins above replacement, per FanGraphs, over the final 48 games, culminating in winning the World Series.

The cons

Frankly, any change of this level hinders the routines from their established starters.

“I think it's also good that we stay on our same type of schedule,” George Kirby said. “I like the [five-day]. We all have good starters and like to keep them all in. At the end of the day, I think five is better. But whatever their call is.”

The rookies have also become accustomed to such a routine.

“I've gotten to where I like going on five,” Miller said. “But it's like, whenever I do go five days it's like the most I've had is like two five days in a row, and then it’d be a sixth day or an off-day. It hasn’t been that big of an adjustment. Every now and then in the Minor Leagues, I’d go five days. But I’ve grown to where I like the five days.”

Added Woo: “After the first couple starts, you kind of just get used to the schedule. Obviously, it's one less day in rotation [compared to] Minor Leagues, but I feel like I was able to get in a good groove.”

The long-term outlook

As Dipoto mentioned, this is only temporary -- potentially until the Mariners get through the September gauntlet.

If they were to reach the postseason, the rotation would likely tighten to three, just as it did last year along with most clubs, at least for the early rounds.

“We control our own destiny. ... We have a schedule over the course of the next three or four weeks where, if we do our job, we should come out OK,” Dipoto said.