SEATTLE -- As the Mariners begin moving forward in the rebuilding process for next season, starting pitching figures to be one of their biggest question marks -- and Justus Sheffield's primary goal is to be one of the answers.

The 23-year-old from Tennessee looms prominently in Seattle¡¯s projected rotation after getting his feet wet in the final five weeks of the 2019 season, posting a 5.45 ERA in 33 innings over seven starts.

Outside of one relief appearance in late April when he piggybacked a short start by Yusei Kikuchi, the Mariners kept the kid gloves on Sheffield, delaying his Major League arrival until Aug. 23.

While he had mixed results after his second promotion, the club was encouraged by his swing-and-miss stuff once he began commanding his excellent slider to go along with a mid-90s fastball. Sheffield whiffed eight in 4 1/3 innings against the White Sox on Sept. 15 and closed out his campaign with five frames of one-run ball against the playoff-bound A¡¯s on Sept. 27.

The Mariners love Sheffield's upside potential, believing he has the tools to be a quality Major League starter. Now he just needs to gain the experience and go through the learning curve that every youngster faces when making that jump.

¡°Justus has been just about what you would expect of a ¡®stuffy¡¯ 23-year-old,¡± said general manager Jerry Dipoto. ¡°I think we went 6-2 in the eight games he pitched in and [his final game against Oakland] was probably the least consistent of his outings and he still gave us a chance to win. But we saw physical stuff, a fastball up to 97 [mph], and an out-pitch slider. We saw continued improvement from his changeup. I'm really excited about where he is.¡±

What went right?

Sheffield soaked up his first real MLB experience and took his lessons to heart. He knows now that he needs to continue developing his offspeed offerings to compete at the big league level and to get physically stronger after pushing himself to a career-high 169 innings between the Majors and Minors.

¡°I had some good games, some bad games and some in between,¡± he said. ¡°But for me personally, just getting out there and getting comfortable and learning from each game has been huge. I didn¡¯t think I could learn this much in that short of time. I¡¯m super excited for next year.¡±

Though Sheffield is still looking for his first Major League victory, that was mostly a matter of being limited to four to five innings most outings and not getting much help from the bullpen. He left the game with a lead or a tie in five of his seven starts.

¡°I would hope he¡¯d continue to move in the upward track he¡¯s on right now,¡± said manager Scott Servais. ¡°We¡¯ve seen a lot of good things from him and he continues to learn a lot. The learning is not going to stop.¡±

What went wrong?

After an outstanding spring, Sheffield lost his command when he began the year in Triple-A Tacoma. The Mariners felt he tried to be too fine with his pitches to avoid contact in the homer-happy Pacific Coast League and got away from his strengths.

Sheffield had gone 6-4 with a 2.56 ERA in 20 games (15 starts) for the Yankees¡¯ Triple-A Scranton/Wilkes-Barre club in 2018, with 84 strikeouts and 36 walks in 88 innings, and opened the year as Seattle's No. 1-ranked prospect per MLB Pipeline.

But he went 2-6 with a 6.87 ERA in 13 games (12 starts) for Tacoma, with 48 strikeouts and 41 walks in 55 frames before being demoted to Double-A Arkansas and dropping to No. 9 in Seattle's updated prospect ratings.

Best moment

Sheffield reinforced what the Mariners believe he's capable of doing with a strong outing against the Cubs on Sept. 2, throwing five scoreless innings with seven strikeouts. While he gave up five hits, two walks and hit two batters, Sheffield kept Chicago¡¯s potent lineup at bay with an outstanding 21 swinging strikes in a 91-pitch outing.

The Cubs went 0-for-8 with four strikeouts against Sheffield with runners on base and the 2014 first-round Draft pick relished the chance to shine at Wrigley Field.

¡°I love those moments. I think it¡¯s pretty cool,¡± he said. ¡°It¡¯s not every day somebody can go out there and pitch in a crowd like this. I just wanted to have fun and make the most of it.¡±

2020 outlook

Sheffield, who needs to continue developing his changeup as a third weapon in his arsenal, felt he made large strides with that in his final starts. Now he knows what to expect and will come to Spring Training in February with an expectation of picking up where he left off, starting every fifth day and seeing what he can do with the opportunity.

¡°I would love to see him in our rotation next year and see if he can go out and make as many starts as Marco [Gonzales] did,¡± Servais said. ¡°That would be a goal. It will be challenging, going through a full Major League season, but we're going to need young guys to do it.

"It¡¯s a big offseason for a lot of players and how they go about their training and where their mindset is at now that they¡¯ve got a taste of it and understand what it all entails.¡±