SAN DIEGO -- A Mariners team that has been exploring every avenue to beef up its bullpen over the past year added an intriguing power arm in Thursday's Rule 5 Draft, selecting 24-year-old right-hander Yohan Ramirez out of the Astros' organization with the sixth pick.

Ramirez must be kept on Seattle's 26-man Major League roster for the entire 2020 season, including at least 90 days on the active list -- as opposed to the injured list -- or be offered back to the Astros.

General manager Jerry Dipoto and his staff identified two pitchers they were interested in from the Rule 5 pool and jumped at the opportunity when Ramirez was still on the board.

"As we continue to march toward our 2020-21 goals of where our Major League team is, Jerry has talked about the opportunities we can give," said Mariners scouting director Tom Allison. "And this is another example of that. Trying to add real big upside power pitches to a young man who hasn't had a chance to pitch above the Double-A level. We're excited for that."

Ramirez features a mid-90s fastball that has reached the 97-98 mph range at times, along with a hard slider and changeup. He racked up 158 strikeouts in 106 innings between Class A Advanced Fayetteville and Double-A Corpus Christi last season, but also issued 74 walks.

His 13.4 strikeouts-per-nine innings ratio was third among all Minor League pitchers with a minimum of 100 innings pitched, and he was fifth in opponent batting average at .169.

Astros special assistant Kevin Goldstein said Ramirez's situation simply comes down to his ability to command the strike zone.

"He throws really hard, really good slider," Goldstein said. "You guys don't need to be super advanced baseball people to look at the page on Baseball Reference and see all the walks. It's going to come down to going to Arizona this spring and seeing if he can throw more strikes. The stuff is there. It's some of the better stuff we have in the system. There's just control issues."

The Mariners hope to harness his command and are intrigued by his potential as a multi-inning reliever, with the Dominican Republic native having started 15 games last season.

"This is a guy who really lit up everybody with his physical ability," Allison said. "We were able to piece together what type of human he is and work ethic, and [we] feel you add those things together and give him to our staff and he has a chance to be a really successful big leaguer."

The Mariners didn't lose anyone in the Draft, which is open to players who are not on 40-man rosters and have been in pro ball for four or more seasons after being signed out of college, or five-plus seasons after being signed at 18 or younger.

Mariners select three in Minor League phase

Seattle added three players and didn't lose any in the Triple-A portion of the Rule 5 Draft, taking catcher Brian O'Keefe from the Cardinals in the first round, outfielder Jhonny Santos from the Marlins in the second and first baseman Dariel Gomez from the Cardinals in the third.

Allison said O'Keefe will be assigned to Double-A Arkansas, where the 26-year-old will provide a veteran presence to work with young Cal Raleigh, the Mariners' No. 7 prospect. O'Keefe posted a .229/.319/.389 line with 13 homers and 40 RBIs in 88 games with Double-A Springfield last year.

"O'Keefe was a catcher we saw a lot of in the Texas League," Allison said. "With Cal Raleigh slated to be the everyday catcher there, it's an opportunity to have an experienced catcher provide not only leadership as a veteran to the league, but his framing and game-calling skills are something that stood out to us."

Santos, 23, is a promising defensive outfielder who needs to develop with the bat, having hit .241 with four homers and 12 stolen bases in 55 games for Class A Advanced Jupiter in the Marlins' system.

Gomez is a 6-foot-4, 190-pound first baseman with power potential. The 23-year-old batted .262 with six homers and 27 RBIs in 63 games in Class A Short Season for the Cardinals.

Players added in the Minor League phase can be assigned to any level of their new organization, but must remain in the system for the full season.