It’s the eve of the 2025 Rule 5 draft protection deadline. The Miami Marlins have until 6 p.m. ET on Tuesday to decide which of their Rule 5-eligible prospects to select to their 40-man roster. Joe Mack is in a tier of his own—selecting his contract has been an inevitability for months. I placed Josh White in his own tier right below Mack because although his performance and quality of stuff should also make him a no-brainer, it’s especially tricky to evaluate and appraise relief-only prospects.
The lone resident of the third tier is fellow right-handed reliever William Kempner. In his first season with the Marlins organization, Kempner climbed from High-A to Triple-A, posting a 2.26 ERA across 67 ⅔ innings pitched while striking out one-third of opposing batters. He allowed only two home runs.
The antithesis of White, who has an extreme over-the-top delivery, Kempner practically throws sidearm. It’s a helpless feeling for righty batters who have to guess whether they’re getting a sinker that averages 18 inches of armside run or a slider breaking 14 inches in the opposite direction, as illustrated below in a matchup against former major leaguer Corey Julks:
When facing lefties, the 24-year-old leans heavily on his four-seam fastball, which averaged 94.9 mph in his Triple-A appearances and topped out at 98.3 mph.
On the concerning side, Kempner’s control eroded as the level of competition increased:
9.0 BB% and 3 HBP at High-A (111 batters faced)
15.6 BB% and 2 HBP at Double-A (96 batters faced)
19.7Â BB% and 3 HBP at Triple-A (76 batters faced)
Kempner was sidelined for the entire 2024 minor league season while recovering from foot surgery. That’s been his only significant injury absence dating back to the beginning of his collegiate career.
If we assume both Mack and White are being protected, that means protecting Kempner would require a corresponding 40-man roster move. I have a hunch we’d see Zach Brzykcy—claimed from the Washington Nationals earlier this month—designated for assignment with the intent of passing him through waivers and outrighting him to the minors. That’s precisely how the Marlins handled Christian Roa at this stage of the 2024-25 offseason, for what it’s worth. Another possibility would be giving Andrew Nardi his pink slip a few days in advance of Friday’s tender deadline.
Upon closer inspection, I’m personally not as high on Kempner as I was when crafting my Marlins offseason blueprint. I still recommend that the club select him to the 40-man, but it would be unreasonable to count on him pitching meaningful MLB innings in 2026. Even if he initially succeeds, I wonder how long that would last when big leaguers have the technology to take pregame practice swings against his unconventional release point.
The key question is whether the Marlins have the ambition and resources to add multiple experienced arms to their bullpen this winter. It’d be far easier to stomach likely losing Kempner in the Rule 5 if it’s part of the process to make room for trustworthy veterans.
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