September brings roster expansion in Major League Baseball, which means contending and rebuilding teams alike will bring up some of their top prospects for playing time to end the season.
Not only are rosters increasing from 26 to 28 in less than two weeks, but since there are fewer than 45 days left in the regular season, any player called up will not lose his prospect eligibility based on days in the big leagues.
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So which players could be making their debuts in the closing weeks of the regular season?
On the latest episode of the MLB Pipeline Podcast, Jim Callis and Jonathan Mayo took turns picking Top 100 prospects based on the likelihood of them making it to The Show this year. Here are their picks, with the most likely players listed first.
The No. 3 overall pick in last year's Draft has become the top pitching prospect in the game. With four above-average-or-better pitches and plus control, he leads all qualified full-season Minor Leaguers in ERA (1.06), strikeout rate (40.5 percent), strikeout-to-walk ratio (10.4), average-against (.147) and WHIP (0.64).
Callis: "He's having one of the best pitching seasons in Minor League history. He's been doing this from Day 1. He's just gone out and dominated. This guy could get outs in the big leagues right now. Stop screwing around, and I know you're going to have to displace somebody in the rotation, but I'm kind of surprised. They're just wasting his time in Double-A at this point."
Ralphy Velazquez, 1B/OF, Guardians (CLE No. 2/MLB No. 32)
Originally drafted as a catcher, the 2023 first-rounder has developed into the game's top-ranked first-base prospect. The left-handed hitter combines natural strength with dynamic bat speed and a penchant for pulling the ball in the air to deliver plenty of home runs, and he has had almost no platoon split in the upper Minors.
Mayo: "He's their type in terms of another left-handed bat. He's now really just heating up. It's the second straight year with 20+ home runs. If you look at who has been playing first base for Cleveland, it's not like they're getting production there, and he can play a corner outfield spot, so I feel like they should strike while his bat is hot and give him his first opportunity in the big leagues. He could give them a boost right away with how he's been swinging the bat."
Gage Wood, RHP, Phillies (PHI No. 1/MLB No. 33)
The 26th pick in last year's Draft started the season at Single-A and jumped straight to Double-A, where he's dominated with his 70-grade fastball. He's also impressed with his control, as evidenced by his 27.1 percent K/BB rate, which ranks only behind Anderson, Christian Zazueta (LAD No. 5/MLB No. 36) and Seth Hernandez (PIT No. 1/MLB No. 7) among Minor Leaguers with at least 70 innings.
Callis: "Even if you don't want to put him in the rotation right away, you bring in Gage Wood and let him pitch four innings as a bulk reliever behind [Andrew Painter and Aaron Nola]. He could really help that Philly staff. When you have guys who've shown their ability to throw strikes and miss bats in Double-A, I just wouldn't mess around anymore. This Phillies team, their window is not going to be open too much longer, so I would do everything to maximize my chances of winning this year."
Walker Jenkins, OF, Twins (MIN No. 1/MLB No. 15)
The Twins drafted Jenkins fifth overall in 2023, and he's dazzled when healthy with five above-average-or-better tools and a career .298/.399/.475 slash line. The North Carolina native ranks in the 78th percentile at Triple-A with a 43.9 percent hard-hit rate and plays quality defense in center field, although he has plenty of arm for right.
Mayo: "The thing that is remarkable about Walker Jenkins is his ability to produce, even though he continues to miss a lot of time. This time it was a shoulder injury after crashing into an outfield wall. He has yet to play 85 games in a season, but here he is swinging a really good bat. The Twins brought Kaelen Culpepper (MIN No. 3/MLB No. 34) up to try to give them a boost as they're hanging around the AL Central and the Wild Card races, and I could see him getting an opportunity soon to try to give them a little bit of an offensive jolt."
Theo Gillen, OF, Rays (TB No. 2/MLB No. 12)
After a wrist injury ended his 2025 campaign in mid-August, Gillen has returned with elite production at Double-A. He's one of eight qualified full-season Minor Leaguers with an OPS over 1.000, and he's closing in on a 20/40 season. The 2024 first-rounder is tooled up and showing strong plate discipline to boot.
Callis: "Theo Gillen might be the best hitter in the Minor Leagues this season when all is said and done. They've got the luxury of, yes, you're a playoff team, but since you've wrapped up that berth, you've got about 38, 40 games left in the regular season at this point. Figure out, are we better with him in the lineup? If we use him in center field, is he ready for that? Can he handle it?"
Michael Arroyo, OF/2B, Mariners (SEA No. 3/MLB No. 45)
The Colombia native is physically mature for a 21-year-old who just made it to Triple-A, and he'll routinely barrel the ball from the right side of the plate while rarely striking out. After signing as a shortstop, he has split his time between second base and left field in the upper Minors.
Mayo: "I don't know where he would fit in exactly, but the guy hits and he's been hitting in Triple-A. Maybe they could move him back and forth like they've been having him do in the Minor Leagues. He was one of these guys that was hard to figure out where to put in the Top 100, because where does he play? How does he profile? But what he does is hit, and they could use some more offense to help back up that very deep pitching."
River Ryan, RHP, Tigers (DET No. 4/MLB No. 66)
The Tigers acquired Ryan from the Dodgers as part of the return for Tarik Skubal, and the right-hander, who can touch triple digits with his fastball, immediately slotted in as the club's top pitching prospect. Working his way back from Tommy John surgery in August 2024, the 28-year-old is currently sidelined with a hamstring injury.
Callis: "If you look just in terms of pure stuff, he has one of the deepest and best repertoires in the Minor Leagues. The Tigers are definitely in contention. They don't really have a fifth starter, per se. They just added Jackson Jobe to the rotation. He's coming off of Tommy John surgery. His first two starts weren't great, but they need rotation help. So I think as soon as River Ryan's healthy, he's got the stuff to make a difference in Detroit if he's given the opportunity."
At just 20 years old, Arias is the youngest player in the Triple-A International League and has retained the sterling defensive reputation that made him a top 2023 international signee out of Venezuela. He has grown into impressive offensive tools since, led by advanced bat-to-ball skills and improved bat speed.
Mayo: "What he's done from an impact standpoint has been one of the big reasons why he's moved up the list as much as he has. He's super young, he's showing impact and power since he's been in Triple-A. He does not swing and miss much. He's played mostly shortstop, but he has some experience at second base. I think they would be comfortable with him at either spot."
One of the top power hitters in the Minors, Walcott has lived up to the hype since signing for $3.2 million out of the Bahamas in 2023. At age 19, he was the youngest qualified hitter at Double-A last year and returned to the level in mid-July after recovering from internal brace surgery.
Callis: "He's got huge power. He's got huge arm strength. He's pretty much just DHing at this point, but he's been on fire since he's returned to Double-A. The Rangers have been platooning Joc Pederson with Justin Foscue at DH, and I'm at the point where I would call Sebastian Walcott up and give him the opportunity to get some at-bats. That one's more of a pipe dream compared to some of these other ones I've said, but he's just so precocious and he's hit everywhere he's gone."
Made is the complete package with plus tools across the board, save for an above-average glove. The 19-year-old switch-hitter has thrived at Double-A against much older competition and has shown little platoon split historically, with impressive bat speed from both sides of the plate.
Mayo: "No. 1 prospect in baseball, you always want that guy to get called up. The Brewers are not afraid to call guys up quickly or push them from Double-A or as a teenager. He's had a really good year in Double-A, just a solid all-around player. I don't know what the avenue would be to call him up, maybe third base. He's the kind of guy that, in the previous system when you had larger September callups, maybe you would see him make an impact pinch-hitting, maybe DH's occasionally, you give him a game at short."