MIAMI -- As expected, the Marlins added catcher Joe Mack -- their No. 4 prospect and No. 70 overall per MLB Pipeline -- to their 40-man roster ahead of Tuesday’s deadline.
The Marlins also protected right-handed relievers William Kempner and Josh White from the Rule 5 Draft, which will be held on Dec. 10 in Orlando, Fla. Since Miami had only two open spots on the 40-man roster, the organization cleared space by designating for assignment Joey Wiemer.
Mack, who turns 23 on Dec. 27, was the only player on the Marlins' Top 30 Prospects list who was eligible for this year’s Rule 5 Draft.
"Really excited about those guys," president of baseball operations Peter Bendix said on Friday. "Joe Mack did everything you could want from a 22-year-old catcher. ... Offensively, defensively, as a leader, he's a special player."
After receiving an April 22 promotion from Double-A Pensacola to Jacksonville, Mack slashed .250/.320/.459 with 18 doubles, two triples, 18 homers and 53 RBIs in 99 games for a Jumbo Shrimp squad that captured its first championship. His 21 home runs between Pensacola and Jacksonville ranked fifth among Minor League catchers.
According to MLB Pipeline’s scouting report, Mack’s “quick left-handed swing and strength produce plus raw power that plays to all fields. He works deep counts and doesn’t chase too often … [but] he struggles to deal with breaking balls and changeups,” as evidenced by his increased strikeout rate (27.9%) and a near-career-low walk rate (8.5%).
Despite Mack’s defensive numbers taking a dip from 2024 (one error and seven passed balls) to '25 (11 errors and nine passed balls), the 2024 Minor League Gold Glove Award winner at catcher is still highly regarded. Per MLB Pipeline, Mack is “agile behind the plate and pounces on grounders in front of him. He has soft hands and has developed into a solid receiver and framer as he has gained experience and improved his focus.”
Thanks to a 60-grade (on 20-80 scale) arm, Mack threw out 33% of basestealers (35-of-105) this season.
“For hitting, I would say my strength right now is being able to put the bat on the ball consistently,” Mack said before participating in the 2025 All-Star Futures Game on July 12. “And for catching, I would say framing and my ability to throw people out trying to steal bases.”
Kempner, whom the Marlins acquired from the Giants for international bonus pool money on Jan. 15, compiled a 2.26 ERA and seven saves in 48 appearances across three levels (High-A Beloit, Pensacola and Jacksonville). In 17 innings for Jacksonville, the 24-year-old recorded 13.2 K/9 and 7.9 BB/9. Kempner maintained his fastball velocity in the mid- to upper 90s while maxing out at 99.2 mph on Aug. 28.
"He came over in a relatively small deal and really impressed us from Day 1 and just kept getting better," Bendix said.
Miami selected White, who turns 25 on Monday, in the fifth round of the 2022 MLB Draft out of the University of California. He turned into one of the organization’s top relievers, with a 1.86 ERA in 45 appearances (43 in relief) between Double-A and Triple-A. With the Jumbo Shrimp, White tallied four saves, a 0.97 WHIP and 13.0 K/9 and 2.5 BB/9 rates. He pairs a mid-90s fastball with a slider.
"Josh White made incredible improvement, really big testament to him that he put himself in position to be added to the roster," Bendix said.
Being added to the 40-man roster comes with an automatic invitation to big league camp. Mack, who was a non-roster invitee last spring, will split reps with second-year players Agustín Ramírez and Liam Hicks, barring any further changes. Kempner appeared in four Grapefruit League games last spring, while White faced two batters in one outing.
Outside of that trio, the Marlins left several prospects unprotected, including a pair acquired in trades over the past two years.
First baseman Nathan Martorella (acquired in the Luis Arraez trade with the Padres in 2024) finished '25 at Jacksonville, where he posted a .631 OPS in 39 games. Center fielder Andrew Pintar (acquired in the A.J. Puk trade with the A's in '24) recorded a .722 OPS in 84 games at Triple-A. And third baseman/outfielder Jacob Berry, the sixth overall pick in the 2022 MLB Draft, turned out to be the walk-off hero in the Triple-A national title game. Though Berry has a career .695 OPS in 396 games in the Minors, he slashed .304/.387/.447 from May 14-Sept. 25.
Wiemer, whom Miami claimed off waivers from Kansas City on Aug. 4, went 13-for-55 (.236) in parts of 27 games for the Marlins. The 26-year-old was out of Minor League options. With Wiemer’s departure, Heriberto Hernández and Dane Myers are the only right-handed-hitting outfielders on the 40-man roster. Gold Glove Award-winning utility player Javier Sanoja would be another option.