SEATTLE -- Throughout the Mariners’ Virtual Baseball Bash the past two weeks, the club made it clear: the six-man rotation that it deployed last season will remain in place in 2021.

Much has gone into that decision -- most chiefly, the health and workload limitations of the Mariners’ young staff, especially given that even the most established and experienced will be working their way back from a 60-game schedule to a sixth-month marathon. And while there hasn’t been precedent of a six-man unit over an entire 162-game season in recent years, the Mariners are committed to its benefits.

“The six-man rotation allows for two bullpen days. It allows for two work days between [starts], and when your pitchers are young, those days are really important,” Mariners general manager Jerry Dipoto said. “It allows for pitch development and pitch shaping. It allows for training on delivery refinement and command. It’s not just about getting out and performing. It’s very much still a development program. We feel like that’s a huge advantage for such a young staff.”

Beyond Marco Gonzales and Yusei Kikuchi, the Mariners are dealing with a very green group of arms. Justus Sheffield compiled a 3.58 ERA in a stellar rookie season, but he made just 10 starts; same for Justin Dunn, who pitched at least six innings four times but also failed to go more than four innings four times.

Nick Margevicius was a welcome surprise, but his sample size in 2020 was just seven starts. And Kikuchi always pitched on at least one extra day’s rest in ‘20, a practice that the Mariners reverted to since it was one that Kikuchi was accustomed to in Japan.

Dipoto has touted newly acquired Chris Flexen, who pitched in Korea last season, but the righty’s big league numbers with the Mets (3-11, 8.07 ERA, minus-2.2 wins above replacement) paint an uncertain picture.

Mariners’ starting rotation (career-high innings)

1. Marco Gonzales (203 in 2019)
2. Justus Sheffield (133 in ’19; Minors)
3. Yusei Kikuchi (187 2/3 in ‘17; Japan)
4. Justin Dunn (135 1/3 in ’18; Minors)
5. Nick Margevicius (135 in ’18; Minors)
6. Chris Flexen (134 in ’16; Minors)

“It was such a young staff and it is still a young staff,” Dipoto said. “And that's an important element here. So with that, and understanding that the highest-volume starters in 2020 were guys that threw 60-80 innings. And that's a that's not a high volume of innings in the grand scheme of things. And we feel like the jump from 60-80 to, say, 140-150, is reasonable. For the guys who might have been at the top end of that scale, to get them into a 160-170 type of range is reasonable. Going beyond that is not likely, and we feel like this allows us to help govern that.”

It all points to Seattle’s rotation being tentative. Even for the most loaded MLB staffs, it’s not out of the realm to use to 10 (or more) starters to get through a six-month season when considering injuries, attrition and other factors. The Nationals, for example -- even with Max Scherzer, Stephen Strasburg and Patrick Corbin -- used 10 starters in their World Series-winning 2019 regular season.

With the 162-game season slated to start on time, that would mean 22 off-days to account for extra rest and turns through, opposed to the six off-days in 2020. And if active rosters return to 26 players, the Mariners would almost certainly use that spot for an additional reliever, which would leave them with six starters, eight relievers, eight starting position players, a bench of just three plus the backup catcher and without a true fourth outfielder.

“If it's possible, we are open to adding another starting pitcher and increasing our depth in the rotation,” Dipoto said, noting that the club has also been engaged in active talks to add a left-handed bat.

Regardless of ambition, injury attrition, performance and other uncertainties suggest that the Mariners will likely need reinforcements at some point.

They’ve made it clear that Logan Gilbert, the club’s No. 4 prospect by MLB Pipeline, will contribute Major League innings this season -- possibly even for a spot to break camp with the big league club. Ljay Newsome made four starts last year and remains on the 40-man roster. So is Robert Dugger, who the club acquired off waivers on Dec. 7 from Miami, where he made eight starts with a 7.40 ERA. Erik Swanson began as a starter in 2019 but has struggled since transitioning to the bullpen.

There are free agents out there, and a source told MLB.com that the club's market has gained some traction over the past two weeks. A reunion with Taijuan Walker would make a lot of sense, and Gonzales has lobbied for a Walker return on Twitter. However, Walker is likely seeking a multiyear deal after re-establishing his value in 2020 by helping the Blue Jays reach the postseason, and it’s unclear if the Seattle is ready to make such a commitment. There’s also the possibility that the Mariners could add a veteran on a Minor League deal to compete for a spot in Spring Training.

“I would love to have him back. I don't know how much headway I can make,” Gonzales said of Walker. “It's not my job to get him here. But on the AL West front, I don't think I'm alone in seeing a window, seeing an opportunity to really make a statement in this division. I think with the young talent that we have a culture that we've got to create here, we are on our way. And that's where the expectation lies right now. I won't settle for anything less.”

Beyond the rotation, there’s a bullpen in play that will be following the six-man unit, one that compiled an American League-worst 5.92 ERA last season. Dipoto bolstered the relief corps by acquiring Rangers closer Rafael Montero and free agent Kenyan Middleton, and re-signing Kendall Graveman, who has unearthed high-90s velocity as a reliever. But part of the bullpen’s overall success will hinge on consistent longevity from the starters.

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The Mariners certainly won’t be the only team that deploys a six-man rotation in 2021. The up-and-coming Tigers have hinted at it, and the Red Sox have talked about managing workloads, which suggests that they may explore the idea. Even the Padres, who acquired Yu Darvish and Blake Snell this offseason, could consider a six-man unit to help them allocate innings given their dearth of starting depth beyond their top tier.

But Seattle is the first club at the forefront of this trend -- and at least the most publicly committed.