SEATTLE -- When asked by a fan on Tuesday who might be the Mariners’ most under-the-radar pitching prospect entering 2021, general manager Jerry Dipoto had a quick reply and thorough answer.

“On a more 40-man, close-to-the-Major-Leagues type of scenario, I think Juan Then is a guy who really has made a ton of progress,” Dipoto said. “And Juan is one of those guys who, while he was at the [alternate training] site last year, as well as the Arizona Instructional League, he is just 21 years old this year.

"There's so much development in front of him, and of all of our 40-man players, he is the guy who has the chance to make the biggest step forward in '21, who already took perhaps the biggest step forward in '20.”

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The Mariners’ No. 14 prospect was added to the 40-man roster on Nov. 20 to protect him from being selected by another club in the Rule 5 Draft in December -- a sign that Seattle valued the right-hander enough to keep him in its long-term plans. Originally signed as an international free agent by Seattle in 2016, he was dealt to the Yankees in ’17 and reacquired in the Edwin Encarnación trade in ’19.

“When they acquired me again ... I said, ‘If they’re asking for me back, it’s because I’m good,’” Then said in Spanish recently to MLB Network during the 30th annual Rookie Program held by MLB and the Players Association. “Those opportunities, I’ve earned them, because I’ve always worked hard and have always been focused. I haven’t changed my personality with anyone. I'm always moving forward.”

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Though his development, like the rest in the club’s farm system, was significantly curtailed due to the cancellation of the Minor League season in 2020, Then utilized the time to polish his slider into a true plus pitch during stints at the alternate training site and in Arizona in the fall, while also showing an uptick on his fastball -- from the mid-90s to approaching 100 mph.

“That’s been two years and counting that I’ve been improving my velocity,” Then said. “I’ve always loved working on the mound and things like that. In Arizona, I went in with a goal that I had to explode. I worked hard in the gym with the guys, always focused.

"Then one day, out of nowhere, [I threw] 99, 100. The team was congratulating me and told me they were going see that, because I’ve always worked hard, and I always think about winning. I never think about losing.”

“Getting to be with him every day and seeing the evolution of his slider, which really wasn’t much of a pitch in Seattle [during Summer Camp], but by the time we left Arizona, it was a plus pitch,” Mariners director of player development Andy McKay said in November. “And he was like 95-99 mph every time with a really good slider, so that was great to see.”

Before joining the Major League club during Summer Camp in Seattle, Then trained in his grandmother’s backyard back home in the Dominican Republic before getting in touch and working out with Orioles second baseman Hanser Alberto.

“He would always tell me, ‘Work hard, because you’re going to have an opportunity now,’” Then said of Alberto. “I always kept myself focused and I never slowed down, as difficult as the situation could get.”

During his last full season in 2019, Then compiled a 2.98 ERA and 48 strikeouts, 13 walks and just two homers allowed in 48 1/3 innings across three Minor League levels, topping out at Class A Advanced. MLB Pipeline scouting reports say that Then has shown a penchant for throwing strikes, though he does struggle with maintaining his arm slot on his delivery at times. There’s hope that will get better as he continues to add strength, and that he can slow down his changeup to separate from his fastball enough for him to remain a starter.

His goal in ’21 is to create a more robust diet plan -- and reach the Majors.

“Now that he's on the 40-man roster and the clock is ticking with [Minor League] options, we would anticipate a pretty quick ascent for Juan. I don't anticipate that being in '21, but I think this time next year, you will be really quite surprised by the physicality of his stuff and how quickly he could come.”