Toolshed: A's Mateo, Fowler surprise with pop

The A's swear they don't prioritize power with their prospects. Recent history, however, tells us they've done an impressive job of taking advantage of the tool.
As in previous seasons, this week's Toolshed uses isolated slugging percentage to measure how productive prospects were in the power department during the 2017 campaign and how that compares to their expected output based on the power-tool grade given to them by MLB.com.

For the uninitiated, ISO is taken by subtracting a player's batting average from their slugging percentage. In other words, a player could bat .500 and therefore have a high slugging percentage of .500, but his .000 ISO would show there was no power in the profile.
Toolshed then takes the mean and standard deviation of ISO and power-tool grades, as doled out by MLB.com. This keeps everything relative and fairly on the same plane. The mean ISO for position players ranked among MLB.com's Top 100 this season was .174, while the standard deviation was .052. The mean power grade was 50.79 on the 20-80 scale and the standard deviation was 5.49. Though "average" power is considered a 50, the higher mean here makes sense as this is not necessarily an average group of prospects. They're going to receive higher tool grades.
Take that all into account, and consider that these were the biggest ISO overperformers. (Note: some numbers may appear off but are results of rounding up.)

Biggest ISO overperformers among top-100 Prospects (min. 100 at-bats)

A few of the names on that list should come as no surprise. Orioles outfielder Austin Hays was a third-round pick in 2016 but ended up leading the Minors with 310 total bases, earning a promotion to Baltimore as a result. Phillies infielder J.P. Crawford used a hot July and August to finish with a career-high 15 homers before getting his own call to the Majors. Francisco Mejía similarly finished with a career-high 14 homers in 92 games at Double-A Akron, shucking the belief that his only offensive tool is his ability to hit for average.
The other two names provide our theme here.
Both Jorge Mateo and Dustin Fowler, along with right-handed pitcher James Kaprielian, were traded from the Yankees to the Athletics in the July 31 deal that sent Sonny Gray the other way. Mateo moved on to Double-A Midland, where he carried over impressive results from Trenton, following the trade. Fowler has yet to don an Athletics affiliate uniform after he suffered a season-ending ruptured right patella tendon in his knee during his Major League debut with the Yankees on June 29.
They both joined an organization that has produced a lot of rookie power of late. A's rookies led the Majors with 66 homers in 2017 and ranked fourth with a collective .205 ISO, despite playing home games at the spacious Oakland Coliseum. Matt Olson made plenty of headlines with 24 homers in 59 games, but Chad Pinder (15 homers) and Matt Chapman (14 homers) added to the long ball tally. 
"I think it's really an element of our scouting department and then a lot of hard work from the strength and conditioning guys to get these players where they've been," A's director of player development Keith Lieppman said. "These are big, strong, really fast-twitch players first and foremost. I really think that comes from drafting well, or in the case of the trade, acquiring guys that are athletic with some strength that can become better hitters once they join us."
"It's always something you look at and consider, but this wasn't necessarily created by design," added Dan Feinstein, an A's assistant general manager in charge of pro scouting. "Chapman, Olson, those guys are different types of players from Fowler. We drafted them thinking they could become power bats. As Fowler comes into his swing, he could add some power, but we don't view him in that same light."
Fowler isn't expected to become the next prodigious slugger headed to Oakland, but the 22-year-old outfielder did take a sizable leap when it came to power in 2017, a leap that enabled him to jump to the No. 66 spot in MLB.com's Top 100 after starting the season unranked overall. Batting from the left side, Fowler was already coming off a bit of a career year in the power department in which he hit 12 homers, had 57 total extra-base hits and produced a .177 ISO over 132 games for Double-A Trenton. 
In 2017, he matched his career high with his 12th homer for Triple-A Scranton/Wilkes-Barre as early as June 20 (64 games into the season) and broke that personal best with a dinger in his final RailRiders at-bat on June 27. According to the system used above, Fowler was meant to be an average power hitter but performed more like a 60-grade slugger. In fact, his .249 ISO at Triple-A ranked seventh among Top-100 prospects this season, beating out noted sluggers such as Lewis Brinson (.231), Ryan McMahon (.228) and Scott Kingery (.227).

Like the rest of the baseball world, the A's took notice and could see a future in which the power tool becomes another weapon in a profile that already includes impressive grades for speed and defensive work in center.
"He's obviously a two-time double-digit home run hitter the last two years, and that's pretty good no matter what," Feinstein said. "We always thought there was some extra-base power there, but he showed more than that before the injury. We're not going to lean on him for home runs, but the ability to drive the ball, especially to the pull side, was impressive."
Mateo's case was more extreme but more complicated.
As good a job as ISO can do to highlight hitters' strengths in terms of power, there are some holes with the stat. Since it's based on slugging percentage, which is based on total bases, it's possible for a speedster capable of turning a single into a double and a double into a triple to produce an impressive slugging percentage and by proxy ISO, despite not showing stellar over-the-wall pop.
Enter Mateo. To be sure, the 22-year-old shortstop set a career high with 12 homers in 129 games between Class A Advanced Tampa, Trenton and Double-A Midland this season, beating out his previous best of eight by exactly 50 percent. That's a major development for a player who'd been given a below-average 40 grade for his power. But this is also a player with 80-grade speed and a decent claim to be the fastest man in the Minors. Mateo tied for the Minor League lead with 18 triples in 2017. Those are extra bases that inflated his slugging percentage and raised his ISO to .192, a number akin to a 50-to-55-grade slugger.
"He just has such a disruptive amount of speed, whether it's at the plate or on the basepaths," Feinstein said. "It influences the ways other teams have to prepare and play against him, and that has a huge impact on everyone else.
"He's such a young player still. But he's already dynamic all around with the things he can do. It's electric, and he's doing it at a premium position. The tools across the board have the chance to be high-caliber. The power's still developing, but we're already seeing that it could translate to double-digit homers down the line."

So the question requires asking -- do the A's plan on asking Mateo to tap more into his power in order to make him an even more well-rounded prospect entering his first full season with the club? They don't feel they need to necessarily.
"I don't know that we've worked on power with him," Lieppman said. "That's something that develops over time. We don't want to get into a situation where we change a guy's swing to accommodate power. ... That might force us to create some bad habits. At this stage with Mateo and others, we'd rather let the power come naturally. If I knew how to just give players power easily, trust me, we'd be doing that."
In the end, Oakland may say it's not targeting adding surprising power to the mix as part of its coming rebuild. But when the organization saw an opportunity to add two dynamic position-player prospects on the upswing this July for the club's former ace on the downswing, they were willing to take their chances.
"As far as what we'll do with them, that's up to our coaches and player-development staff," said Feinstein. "But we didn't acquire them to be middle-of-the-order run producers. They have so much else going on for them. Any type of power they can generate is extra as far as we're concerned."
Here are some of the season's other top performers by ISO and biggest underperformers using power tool grades.

Highest Isos for top-100 Prospects (Min. 100 at-bats)

Biggest ISO underperformers among top-100 Prospects (min. 100 at-bats)