With active rosters growing from 25 to 40, September has long been a month for rookies to get their feet wet in the big leagues. Since Sunday, several high-profile prospects have already been promoted to the majors.
Below is a list of the top-10 prospects added to rosters this month. I have defined prospects as players who still have rookie eligibility. In this list, I also included one player (Mike Zunino) whose DL stint ended on Sept. 1, as it wasn't a coincidence he was activated on the day rosters expanded.
1. Mike Zunino, C, Mariners
Zunino is a guy that it is hard to get excited about in the first game you see him, as some scouts don't see any plus tools. But after a few games he grows on you. He has four above-average tools and moves well for a catcher, with some scouts handing out a few plus grades. He performed better in the minors than most scouts thought he would and hung in there earlier this year when he was rushed before he was ready for the big leagues. He made his major league debut on June 12 and played in 30 games, hitting .235 with two homers and 10 RBI before landing on the disabled list with a broken hamate bone. Activated on Sept. 1, his power may take a little time to come back from the injury, but the upside is an above-average, everyday catcher, which is hard to find in the minors.
2. Kevin Gausman, RHP, Orioles
Gausman was hit around some earlier in the year in his first taste of the big leagues, but it didn't affect his ranking as he just got squared up a few times more than normal with some bad flyball luck. The 6-foot-4 righty has an easy plus fastball/changeup combo and has been making strides with a slider to give him a third above-average pitch. With that third pitch, he has a No. 2-starter upside.
3. Carlos Martinez, RHP, Cardinals
Martinez is an easy pitcher to scout as his strengths and weaknesses show pretty quickly. His fastball is regularly in the upper 90s and he backs it up with a plus curveball. His size (6-0/165 pounds) is a concern, however, and his changeup and command will vary start to start. He's only 21, so there's plenty of time to improve. Future closer looks like the floor for his potential, barring injury.
4. Michael Wacha, RHP, Cardinals
Wacha checks a lot of boxes for what scouts look for in a potential mid-rotation arm. He has the size (6-6/200), a solid delivery and at least average command along with three above-average pitches that all flash plus potential at times. His fastball can straighten out and elevate due to his long limbs, but Wacha is already close to reaching his potential.
5. Nick Castellanos, RF, Tigers
Castellanos was a favorite of scouts in high school and early in the minors as he had the rare athleticism and bat control to hit almost any pitch, while also having the size (6-4/210) to project power coming down the road. While he has above-average raw power, that power hasn't shown up in games as much as scouts hoped it would and he wasn't able to stick at third base. Castellanos is still only 21, so he has time to improve.
6. Erik Johnson, RHP, White Sox
Johnson is an under-the-radar prospect from one of the worst farm systems in baseball. He has quietly improved each year after going in the second round in 2011 and has a good chance to be a solid mid-rotation starter. His heavy low-90's sinker and above-average slider and command give him all the elements to succeed early and be an above-average starter for the struggling White Sox.
7. Michael Choice, RF, A's
The book on Choice when he went 10th overall in 2010 out of UT-Arlington was that he checked all of the boxes for a potential above-average right fielder, but he may not make enough contact for it to matter. As it turned out, Choice is a good enough athlete that he was able to make the adjustments he needed to get his K-rate under control in the upper minors and let his above-average tools play.
8. Billy Hamilton, CF, Reds
Scout.com has been lower than the industry on ranking Hamilton, feeling he's only a one-dimensional speed demon. His performance has regressed at the upper levels to the point where most scouts are in agreement about his upside. Pitchers with advanced stuff give him enough problems that he's making weaker contact in Triple-A and may be more of a role player in the big leagues. Hamilton is also one of the fastest players in the game, so he is still a player to watch.
9. James Paxton, LHP, Mariners
Paxton is a guy scouts really want to like. He is a big lefty (6-4/220) who will flash a plus fastball/curveball combo on his good days, but his changeup and command are both average at best. He turns 25 this offseason and is starting to look more like a No. 4 starter or late-inning reliever, but there's still mid-rotation upside if he can put it all together.
10. Jon Schoop, 2B, Orioles
Only 21, Schoop has been very young for his level the last few years, excusing his league-average performances since his Low-A breakout season. He has a physical build (6-1/195), above-average raw power, the ability to play multiple infield positions and solid feel at the plate. All the elements are there, but it might take a few years to all come together.
After this article was posted, Arizona called up the two top prospects in the upper levels of their system, both with everyday potential and limited risk that would've put them on the tail end of this list. Davidson is a bigger guy that will move to right field or first base down the road but has 20-25 home run power and should hit .260 or .270. Owings has big bat speed, above average foot speed and the ability to stick at short but his lack of plate discipline and size limit his upside.