SEATTLE -- With a growing contingent of highly regarded prospects in their Minor League system, the Mariners moved to preserve that group on Friday by adding outfielder Taylor Trammell and right-handers Juan Then, Wyatt Mills and Sam Delaplane to the 40-man roster.

The moves protect those players in next month’s Rule 5 Draft, as they all would have been eligible to be selected by other teams in the Dec. 10 process. The Mariners’ 40-man roster is now at 39.

• What is the Rule 5 Draft?

“We thought all four were mostly no-brainers, based on either their performance or prospect standing throughout the course of their careers to date,” Mariners general manager Jerry Dipoto said.

All four players are among Seattle’s Top 30 Prospects, per MLB Pipeline, with Trammell ranked No. 5, Then No. 14, Delaplane No. 20 and Mills No. 23. Third baseman Joe Rizzo (No. 21) is the only player among the Mariners’ Top 30 who is becoming Rule 5 eligible that wasn’t protected.

• Mariners' Top 30 Prospects

Players not on 40-man rosters who signed at 18 years or younger and have spent five seasons in the Minors, or those who signed at 19 and older and have four years of experience are eligible to be selected in the Rule 5 Draft. That means college draftees from the 2017 class or high school draftees or international signees from '16 will be eligible this year.

Friday was the deadline to set 40-man rosters prior to the Rule 5 Draft, which will take place as scheduled, but virtually -- along with the rest of the Winter Meetings set for Dec. 7-10.

Mills and Delaplane were both drafted out of college by the Mariners in 2017, while Trammell was a first-round pick out of high school by the Reds in ‘16. Then signed with Seattle in ’16 as a 16-year-old out of the Dominican Republic.

Those four all figure prominently in the Mariners’ future plans. Trammell was acquired from the Padres as the key piece in a seven-player trade on Aug. 31 that sent catcher Austin Nola to San Diego. The 23-year-old joined the Mariners’ alternate training site in Tacoma, Wash., for the final month of the regular season, and he spent the past six weeks taking part in the team’s fall development league program in Arizona.

“He’s shown all the tools we expected,” Dipoto said. “He hits the ball hard, super athlete, excellent range in center field, very selective at the plate. We saw a lot of the things we wanted to see from Taylor.”

• McKay talks Julio, prospects from fall camp

Then is another prominent prospect, a 20-year-old starter who features an upper-90s fastball and a developing slider. The Mariners traded Then to the Yankees in 2018, then reacquired him in ’19 and consider him as part of their up-and-coming wave of starting pitching candidates.

“I’m not sure there is a young player in our system over the last 12-15 months that has taken a bigger step forward than Juan,” Dipoto said. “He’s very young and has pitched sparingly at full-season baseball due to situations outside his control. But we’ve seen not just a creep in velocity. ... We saw Juan as a 17-year-old throwing 87-88 [mph] and occasionally touching 90. Now, he’s hitting 100 with some regularity and sitting in the 96-97 range.”

Mills and Delaplane are two relievers with a strong chance to be part of next season’s bullpen. Mills, a third-round pick out of Gonzaga in 2017, wasn’t part of this year’s 60-player pool during the regular season, but he threw well in the fall developmental program.

“Wyatt has always been a solid performer, but his stuff took to a different level,” Dipoto said. “It was very clear that he spent the summer working on a variety of different things that I’m not sure he’d have the same chance to work on with a normal game schedule, just in terms of building arm strength and the like. It was a real big step for Wyatt.”

Delaplane, a 23rd-round pick out of Eastern Michigan that same season, had expectations of jumping to the Mariners this past season after impressing at Double-A Arkansas in 2019. He spent the regular season at the alternate training site in Tacoma, but he never got the call to the Majors and was one of the players most impacted by MLB’s three-month pandemic shutdown prior to the restart of Summer Camp.

“Sam was one of our players who was really negatively impacted by not having access to what he needed,” Mariners director of player development Andy McKay said. “I’m not saying he didn’t work hard or anything like that; he was just not able to do the things some of the other pitchers were doing. So, he was a bit behind, and the ramp up and ramp down really impacted him. He’s going to be a big part of what we’re doing going forward. This was just an anomaly year.”