Editor’s note: This is one in a weekly or semiweekly series called “Player Plan,” analyzing key Mariners players or prospects, looking back on their 2020 seasons and ahead to the offseason and 2021.

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On March 8, when fans were still allowed and COVID-19 wasn’t a part of everyone’s hourly vocabulary, Justus Sheffield pitched three innings, allowing one run on three hits with no walks and five strikeouts against the Giants at Scottsdale Stadium.

Unbeknown to those outside of a four- to five-person conversation that included pitching coach Pete Woodworth, catcher Tom Murphy and Mariners pitching analysts, Sheffield’s plan that day was to test out using a two-seam grip for his primary fastball and scrap the four-seam grip he’d always used to reach this level of baseball.

This change was a major ask, but everyone suggesting it believed it might solve the command inconsistency and pitch inefficiency that plagued the young lefty at times.

Sheffield experimented while playing catch at first, used it in bullpens and finally felt comfortable to debut it in a game.

“I started messing with it and I really liked it,” he said after the start. “It helped me mentally to stay closed in my mechanics because you are thinking you have to stay closed to get that pitch sinking and moving. I’m a pitcher that if I fly open, it’s not a good day. This keeps me on track and closed.”

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And in a game situation, the natural feel — and the results — were noticeable.  

“He just trusted something he could do naturally,” Woodworth said postgame. “ … That was the best fastball and fastball command I’ve seen him have.”

From that day forward, the two-seam became Sheffield’s primary fastball. And that change helped propel him to a solid 2020 season where he cemented a spot in Seattle’s starting rotation moving forward, assuaged any doubts about his status as a key piece of the Mariners rebuild and performed well enough to possibly be one of three finalists for the American League Rookie of the Year award.

“His performance was so far ahead of where he was at this time a year ago,” said general manager Jerry Dipoto. “The personal growth, the strike throwing, getting ahead of hitters and then learning to finish them off and then on days when he didn’t have his best stuff or the best precision, grinding his way through it like veteran winners do. I thought Shef had an awesome year.”

Looking back at 2020

The pre-COVID plan was for Sheffield to spend an entire season in a five-pitcher rotation, racking up 28 to 30 starts if he stayed healthy. But when baseball shut down for three-plus months and came back with a 60-game schedule, Dipoto shifted to a six-pitcher rotation in hopes of keeping starters healthy for this season and ready for next season.

In his 10 starts, Sheffield posted a 4-3 record with a 3.58 ERA. In 55 1/3 innings, he struck out 48 batters and walked 20 and emerged as the second-best starter in the rotation with a 1.6 FanGraphs Wins Above Replacement (WAR), which was the best of AL rookies. He posted six quality starts (six innings pitched, three runs or fewer allowed), which was the most among AL rookies. In six of his final eight starts, he pitched at least six innings, allowing two runs or fewer, posting a 4-1 record and a 2.64 ERA.

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Beyond the results, Sheffield’s competitive nature blossomed while his energy was harnessed. He learned how to give his team a winnable start when his command was off or his mechanics weren’t precise. And he was able to make in-game adjustments, which he couldn’t do during a disappointing 2019 season when he was demoted from Triple A to Double A and left many wondering if he’d ever reach the expectations that came with his prospect status.

“A year ago, I’m probably still out there searching, trying to figure out what’s going on,” he said.

Manager Scott Servais had a simple assessment: “He understands who he is now as a pitcher.”

And just who is that?

“When I was throwing my four-seam and coming up, I always thought I was a power pitcher and I was just gonna blow doors off guys, you know?” Sheffield said. “It was just — here it is. But as I kind of got to the big leagues the past couple of years and being up here pitching, I noticed that these guys can hit 97 to 100 miles an hour like it’s 90 miles an hour. So especially for me, I want to be a starting pitcher so I’m going to need to learn how to change speeds, working inside and outside of the plate. … Another main thing is trusting defense is behind me.”

In 2019, Sheffield threw 322 four-seam fastballs at an average of 92.8 mph, which were 47.8% of his pitches thrown. Opposing hitters batted .299 with a .507 slugging percentage vs. the pitch. This season, he threw a total 417 two-seam fastballs an average of 91.8 mph, which were 47.4% of his total pitches. And while opposing hitters batted .304 against the pitch, it generated just a .359 slugging percentage. Per FanGraphs pitch values, Sheffield’s four-seam fastball had a minus-3.5 value (zero is average) in 2019, while in 2020, the two-seam fastball provided a 0.8 value.

But more importantly, the two-seam fastball provided consistency, allowing Sheffield to get ahead in counts more and use his slider as an out pitch out of the zone. It made the slider value increase from minus-3.8 in 2019 to 4.5 in 2020. Opposing hitters batted .192 vs. the 295 sliders thrown in 2020 with a .219 slugging percentage.

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Offseason focus

Sheffield’s immediate plans included a meal that wasn’t room service or prepared bag lunch from the team.

“I can’t wait to get home for my grandma’s squash casserole,” he said. “That’s my favorite dish that she makes. You know just some good, home-cooked Southern food. I can’t wait to get home and devour some of that, get fat and then run it off in the offseason. I will splurge for a few weeks and then get back in the gym.”

Sheffield will lift, condition and prepare for a 30-start season in 2021. But he knows the Mariners will likely go with a six-pitcher rotation again as a way to protect the starters because they pitched on a limited basis in 2020.

Sheffield is trying to refine the command of his two-seamer to both sides of the plate. He also wants to revive the four-seam fastball for special occasions, particularly getting it to ride up and out of the strike zone. It wouldn’t be used often, perhaps once or twice a game.

But his main focus will be on developing a changeup as a more-effective pitch. He knows that being a 2½-pitch starter just doesn’t work in the long term.

With teams expected to load up on right-handed hitters to face him, and his habit of burying the slider on the back foot for swings and misses, a changeup with a fading/sinking movement away would be an ideal counter.

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“Oh 100%,” he said. “Now that I’m pounding more than inside part of the zone, I have that slider in the back of their head going in, but if I have that third option of being able to throw that slower changeup out and away, it should get swings and misses.”

A look ahead to 2021

A year ago at this time, Sheffield was coming off a lackluster season and his status as a key member of the rotation and the rebuild was in some doubt, even in his own mind.

But now Sheffield is locked into that six-pitcher rotation that will feature Marco Gonzales, Yusei Kikuchi, Justin Dunn and two other starters. The Mariners believe that having a full offseason with the two-seam as his primary fastball will only build on Sheffield’s confidence.

“I wish I would’ve known a little sooner to be able to pick up that pitch and starting throwing it,” he said.

More importantly, they love the shift in his mentality and how it progressed into the season. Sheffield sat in a few of Gonzales’ pre-start meetings and really started adopting the mentality of retiring hitters in three pitches or less.

“It’s been a good year, definitely,” Sheffield said. “I’ve learned a lot and look forward to continuing to learn. I can feel the role of being a legitimate starting pitcher.”