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Lauren analyzes top fantasy baseball prospects risers, fallers, sleepers and busts for 2025. Her MLB rookies report for Week 10 and recently promoted prospects.
Welcome to the latest edition of RotoBaller’s Rookie Report for Week 10 of the 2025 season — your go-to source for tracking the rise of MLB’s most promising young talent. In this issue, we’re spotlighting three players who’ve either recently broken into the big leagues or are still carrying rookie status as the 2025 season gets underway.
In our previous report, we checked in on Matt Shaw, Luke Keaschall, and Agustin Ramirez — each flashing potential while navigating the early hurdles of their pro careers.
This week, the focus shifts to Marcelo Mayer, Denzel Clarke, and Robert Hassell III. We’ll dive into their recent performances and assess what their continued growth could mean for their respective big-league clubs.
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Marcelo Mayer, 3B/SS, Boston Red Sox
Two of the Red Sox's "big three" are now in the majors following Mayer's promotion on May 24. With Boston struggling and falling deeper below the .500 mark and in the American League East standings, it shouldn't be too long before we see Roman Anthony, too. And let's face it -- the Red Sox desperately need a spark. Mayer aims to offer Boston some hope for the future in the coming months.
Track Record:
Mayer was selected fourth overall in the 2021 MLB Draft out of Eastlake High School in California. He immediately got off to a roaring start in the rookie ball that summer, posting a .817 OPS in 26 games.
Mayer followed that up in 2022 with a .280/.399/.489 slash line and 13 homers between Low-A and High-A. By then, the league began taking notice of him, ranking Mayer No. 14 on MLB.com's Top 100 Prospects list.
Now No. 9 in the top 100, he was promoted to Double-A midway through the 2023 campaign but batted just .189 in 43 games to end the year. However, an injury sidelined him for most of August.
In 2024, Mayer appeared in 77 more Double-A games, looking more like his former self. He notched 36 extra-base hits, 38 RBI, 30 walks, and 13 stolen bases, striking out 66 times in 335 plate appearances.
Mayer once again dealt with a couple of injuries, though, setting back his highly anticipated MLB debut. He dropped out of the top-10 prospects, coming in at No. 15, but returned in 2025, at No. 8. An .818 OPS and nine homers in 43 games at Triple-A were all the Red Sox needed to see before finally calling up Mayer to the majors in May.
Outlook:
Through five games with the Red Sox, Mayer has looked the part. The 22-year-old has seemed unfazed by big-league pitching, displaying a maturity about him, even batting in the clean-up spot. His quick, left-handed swing allows him to make contact against high-velocity pitches, something the rest of Boston's lineup grapples with.
Mayer has doubled twice and totaled seven bases in 20 plate appearances thus far, with the below hit coming off the bat at 105.1 mph, his hardest-hit ball that resulted in a hit since his debut.
Marcelo Mayer’s first double of his career.
Just such a smooth and effortless swing. pic.twitter.com/ImtqzgViXB
— Tyler Milliken (@tylermilliken_) May 25, 2025
He hasn't looked outmatched in a small sample size, unlike Kristian Campbell, who has appeared overwhelmed at the plate at times early in his rookie campaign. It makes sense -- Mayer spent much more time in the minors than Campbell. That said, Mayer has cons, too, with an above-average groundball rate and tendency to drive the ball rather than lift it.
Per his scouting report, he also must adjust to non-fastball pitches (1.019 OPS against heaters in 2024, .690 vs. everything else) and same-side pitchers (.896 OPS against righties, .674 vs. lefties). Mayer has all of the tools to succeed at the MLB level. It's only a matter of time before he, Campbell, and Anthony are the faces of the Red Sox.
Denzel Clarke, OF, Athletics
The son of a former Olympian and cousin of major leaguers Josh and Bo Naylor, the Athletics' No. 5 prospect Clarke made his debut on May 23. The team he's employed by is fitting for Clarke, given that he comes from an athletic family and inherited that natural ability, hence why he profiles well in center field. Especially with his large frame at 6-foot-4, 220 pounds, he offers size, power, and speed.
Track Record:
Clarke's scouting report notes that he didn't consistently take up baseball until he was 10 years old, but since then, it has been his life. Taken by the A's in the fourth round of the 2021 draft out of Cal State Northridge, it hasn't always been a smooth ride for him during his pro career. Across Low-A and High-A in 2022, Clarke hit 15 homers, drove in 47 runs, walked 56 times, and stole 30 bases, culminating in a .834 OPS. One downside was that he struck out 135 times in 411 plate appearances.
The Canadian outfielder then spent 2023 and 2024 in Double-A, where he posted a slash line of .266/.355/.463, accompanied by 69 extra-base hits, 96 RBI, 47 stolen bases, 76 walks, and 228 strikeouts in 180 games. Clarke began the 2025 campaign in Triple-A, and in 31 games, had eight doubles, three triples, 23 walks, and seven stolen bags. To shake things up with the major league squad, the A's promoted Clarke, demoted JJ Bleday, and designated Seth Brown for assignment.
Outlook:
Clarke has gotten into six games thus far, and he's left much to be desired. He has just one hit and 15 strikeouts in 19 plate appearances. This isn't new for Clarke -- he's a streaky hitter with swing-and-miss tendencies, resulting in his high strikeout rate. It's possible that he was promoted too early and needed more time in Triple-A.
Save that ball!!!
First Major League hit for Denzel Clarke 🤩 pic.twitter.com/ctadJoMsCk
— Athletics (@Athletics) May 28, 2025
It seems for now that the Athletics are willing to let him play through it. Clarke has long balanced out his strikeouts with his ability to draw walks. Just getting on base, allowing him to display his speed, will give him confidence. Once that happens, the extra-base hits are sure to follow, especially in the A's home ballpark.
Robert Hassell III, OF, Washington Nationals
The 2022 Juan Soto trade has begun to come full circle for the Nationals, as CJ Abrams, James Wood, MacKenzie Gore, and now Robert Hassell III are all in the majors and coming into their own as ballplayers. Wood and Gore have been particularly impressive, while Hassell just debuted on May 22, nearly five years after he was selected eighth overall in the 2020 MLB Draft by the San Diego Padres out of high school.
Track Record:
When Washington received Hassell, he was with the Padres' High-A affiliate, where he was slashing .299/.379/.467 with 19 doubles, 10 homers, 55 RBI and 20 stolen bases. Following the trade, he joined the Nats' High-A affiliate but was promoted to Double-A after just 10 games. He scuffled in 108 at-bats, producing a .607 OPS.
A broken hamate bone in the Arizona Fall League in 2022 forced Hassell to begin 2023 on the injured list. He completed a rehab assignment in Low-A before rejoining the Double-A squad. In 476 plate appearances, he had 24 extra-base hits, a .640 OPS, and a career-high 152 strikeouts, where he struggled against breaking pitches. 2024 saw Hassell pass through three levels, reaching Triple-A in September. Hassell's first stint there wasn't great, posting a .125/.188/.156 batting line in 17 games.
But after a solid spring training in 2025, in which he batted .370, the Nationals decided to start him at Triple-A once again. Hassell fared much better, and in 43 games, slashed .288/.337/.405 with seven doubles, four homers, 24 RBI, and nine stolen bases.
Outlook:
Although still just 23, it's been a long road for Hassell. When healthy, he's a strong, left-handed hitter who boasts speed and contact skills. His injury history has raised questions about his power tool, but if his brief stint in the majors is any indication, he should be fine.
Robert Hassell has his first three-hit game and the first HR of his major league career.
Have a night, kid! pic.twitter.com/wdZ1QDexJc
— Grant Paulsen (@granthpaulsen) May 29, 2025
Hassell hit his first career home run on Wednesday against the Seattle Mariners, traveling 416 feet and leaving the bat at 104.3 mph. The rest of his six hits have all been singles, resulting in a .240 batting average. He has yet to draw a free pass, striking out seven times and stealing a base in 27 plate appearances. If he can maintain some semblance of pop in his bat, the Nationals could have one of baseball's strongest outfields in a few years, with Hassell, Wood, and Dylan Crews.
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