Phillies prospect update: Aidan Miller, George Klassen continue to ‘check the boxes,’ earn promotions

The Phillies promoted a flurry of players to high-A Jersey Shore on Sunday, but two names stuck out: George Klassen and Aidan Miller. Both were taken in the 2023 MLB draft, albeit 166 picks apart.

Miller, a power-hitting shortstop, was drafted in the first round, 27th overall. Klassen, a hard-throwing right-handed starter, was drafted out of the University of Minnesota in the sixth round at No. 193.

They’ve since become two of the most highly touted prospects in the Phillies organization — and good friends. Miller, from Dunedin, Fla., was living at home when he was playing for low-A Clearwater.

Now, he and Klassen will live together in the Lakewood, N.J., area.

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“We were super excited for each other,” said Miller, who turned 20 on Sunday. “We were all told after our game on Sunday. George is a big goofball, but he turns a switch when he’s on the field.”

Miller said Klassen — who hovers in the 96-100 mph range — was “the biggest goofball on the team” when they were in low-A. He liked to do impressions of Sketch, a popular TikTok streamer.

“He does Sketch — the whole thing like, ‘What’s up, brother?’ He does that. Way too much,” Miller said. “That’s probably the best part about him. He’s hilarious. You wouldn’t ever be able to tell that he’d be that good at baseball by his personality. But he’s so good. He throws 100. He’s got a nasty slider and nasty curveball.”

Anyone who has been following Klassen and Miller’s seasons should not be surprised by this promotion. Miller hit .309/.478/.588 in May with three home runs, and is hitting .275/.401/.483 on the season.

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Klassen, 22, had a 0.71 ERA in nine starts (38 innings) at Clearwater. He allowed only one home run.

“I think for both of them, they’ve continued to check the boxes, the things we’ve asked them to do, and I think it’s time to challenge them a little bit more,” said Preston Mattingly, the Phillies’ assistant general manager, player development. “For Aidan, face a higher level of quality of pitching, and for Klassen, face a higher level of hitter. It’s part of their development.”

Here’s a look at why Miller and Klassen have had so much success this season, as well as updates on a few other Phillies prospects throughout the system:

Mattingly said Miller embodies the type of player the Phillies want in their organization because he’s constantly trying to better himself. He has raw talent, but he won’t coast on it.

Miller spent last offseason putting in extra work with the Phillies’ director of hitting development, Luke Murton, on making an adjustment to his setup, so he wouldn’t get beaten by the fastball. He also spent that time refining his game at shortstop with defensive coordinator Adam Everett.

That work with Murton and Everett has continued this season, but Miller has added more to the agenda. He has worked on baserunning with Clearwater bench coach Mycal Jones and Florida Complex League Phillies position coach Gary Cathcart. He’s also been working with mental performance coach Frances Cardenas.

“I think coming into pro ball, everybody is really good at one or a couple things,” Miller said. “The guys that get to that next step are good at everything — even the little things. So every day, I’m looking to improve on those little things, like baserunning jumps.

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“A lot of times, it’s watching video of the opposing pitcher [that we’re going to face] and timing up my jumps based on video from his previous starts. Other times, Gary and Mycal will go over jumps and different situations on the bases during practice. It’s a daily thing. Our coaches in Clearwater do an incredible job.”

Miller suffered a broken hand in his senior year of high school that caused his draft stock to fall, but since then, his prospect stock has gone through the roof. He is now ranked No. 47 on MLB’s top 100 list. He’s walking more, striking out less, and hitting for more power than ever.

He is ready for the next level.

“I think you’re going to get more of the higher-level college pitching in high A, so he’ll be exposed to some of those guys, which is important for him,” Mattingly said. “Those guys usually have a better feel for more breaking stuff, changing speeds, pitching backward, that type of thing.”

The big question for Klassen entering this season was his control. The triple-digit stuff was going to be there regardless. But he allowed more walks (61) than he recorded strikeouts (57) in college. He hit 15 batters and threw 18 wild pitches.

In 2024, he looks like a completely different pitcher. Klassen has allowed just 12 walks to 57 strikeouts. He has hit only one batter and has three wild pitches.

There are a few reasons for this. One is health. Klassen underwent Tommy John surgery in 2021 and did not pitch after he was drafted last year. The Phillies wanted him to add strength. They also wanted him to simplify his delivery, which has made a big difference.

“George has a delivery that produces a lot of power,” said minor league pitching coordinator Travis Hergert. “He’s a very dynamic athlete that can jump out of the gym. So we didn’t want to take away from that. However, we wanted simplify his direction to the plate.

“The changes we’ve made have been to get his direction to not be as rotational. As he goes from his leg lift, we want him working more linear toward the plate and not over-rotating and losing control of the delivery. Paired with work done in the weight room to help him control his powerful movements, he’s able to rotate powerfully but with more control, leading to better command with his high-octane stuff.”

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Another thing that has helped Klassen is having visual cues when he is on the mound. During high performance camp in January, he and Hergert were throwing to a nine-pocket net. Klassen was all over the place.

“So, we asked him, ‘Where do you look or aim when you’re making a pitch?’ And he said he’d never really thought of that,” Hergert said. “So we began to get really specific on where he wanted to aim and start his pitches and where he wanted to then finish.”

Catchers will set up specific targets based on how Klassen’s pitches move. The idea is that the targets will tell him where to start his pitches. If he wants to throw a slider in the zone, the catcher will flash his glove by his left shoulder — signaling to Klassen that’s where the pitch should start.

If he wants to throw a fastball in the zone, the catcher can stay on the plate with a slightly higher glove target.

“Catchers setting up on the plate more, and having specific targets for specific pitches, is something Caleb [Cotham] preaches a lot in the big leagues,” Hergert said. “We carry that all the way down to the minors. It’s a clearer way for us to throw strikes and create strikes.”

Caba, a slick-fielding shortstop out of Bonao, Dominican Republic, is in the midst of his first full season of pro ball. His first season, in 2023 with the Dominican Summer League Phillies, was cut short because of an elbow injury.

This year, he has made the most of his extra playing time. Caba, 18, is hitting .274/.450/.381 with an .831 OPS through 25 games with the FCL Phillies. His approach at the plate is mature for his age — Caba has 26 walks to 15 strikeouts — and the Phillies are confident the power will continue to come.

“I think as he gets older and learns when he can take chances on more pitches and catch the ball out in front, I think he’ll have enough power,” Mattingly said. “Not saying he’s going to be a big power guy. But I think it’s just going to come with age.”

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Rosario, a 19-year-old Dominican catcher out of Santo Domingo, was signed to a minor-league contract in July 2022. He played with the DSL Phillies in 2023 — hitting .264/.402/.319 with a .721 OPS and was promoted to the FCL this year.

Rosario has shown a power bump, hitting .283/.355/.489 with an .844 OPS and three home runs. Mattingly characterized him as “very good” defensively.

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“I think for him, it’s just getting consistent at-bats,” Mattingly said. “Down in the DSL, it’s tough because you have 70 players, and here he’s playing at least four, sometimes five times per week. So he’s been able to get consistent at-bats and then find a consistent approach and have consistent results.”

Unlike Caba, Rosario has a high strikeout rate (27.5%). Mattingly thinks that will lower with experience.

“I think he’s going be a guy that maybe runs a higher strikeout rate, but I think he’s going to have bigger power than most guys, too,” Mattingly said. “So it’s definitely something he can continue to learn. And I think as he learns pitchers’ mistakes and what they’re trying to do to him, and I think the strikeout numbers will come down a little bit.”

Escobar is a 19-year-old infielder out of Caracas, Venezuela. The Phillies have been playing him at second and third base to keep his options open. He signed a minor-league contract in 2022 but has shown quite a bit of pop this year with the FCL Phillies, hitting .338/.495/.481 with a .976 OPS and three home runs through 24 games.

“Last year, per the numbers, he didn’t have a great year, but a lot of the stuff we look at from an underlying standpoint was still pretty good,” Mattingly said. “I know Luke [Murton] and the hitting group have made a conscious effort to adjust a few things, minor things, but I think he’s done a good job at improving. He sees the ball very well, and he has power potential.”

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Escobar has always made contact, but Murton has tried to get him to be behind the ball more, so he can be more impactful at his point of contact.

“He’s always done a good job of getting good pitches to hit,” Murton said. “As he continues to develop, he is becoming more intentional in his approach. He is also learning how to become more connected, which is allowing him to impact the ball with more consistency. [Assistant director, hitting development] Edwar Gonzalez and [FCL hitting coaches] Jake Palomaki and Rafael de Lima have all done a great job in facilitating his development.”

It seemed logical to pair these two, since they came up through the Phillies’ system together and are both pitching at triple-A Lehigh Valley.

Abel, the Phillies’ first-round pick in 2020, has struggled with his command and control this season, and, as a result, hasn’t pitched deep into games lately. The 22-year-old righty has a 6.31 ERA through 10 starts in his first full season at triple A. His strikeout rate has dropped from 25% in 2023 to 18.1% in 2024. His walk rate has risen from 12.5% in 2023 to 15.3% in 2024.

There isn’t one way for a prospect to develop, and Abel still has to figure some things out. But he still is young and has time to do that.

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McGarry, 25, is on the injured list with a sacroiliac joint injury in his lower back. Mattingly said McGarry has been throwing and should be pitching in games soon. He has been working out of the bullpen this season for the first time in his pro career.

So far, it has gone well. McGarry has a 3.57 ERA in 17⅔ innings, has been able to lower his walk rate, and throw more strikes.

“I think this role fits his personality very well,” Mattingly said. “We saw a consistent streak of quality outings. And, honestly, before his back kind of flared up, the strikes were getting better. I think it’s something we’re going to look to build off.”