Breakout sluggers, predicted by fastballs

You can tell a lot about a hitter by how many fastballs pitchers are willing to throw him. The bigger the bat, the more likely it is to see junk. Turns out, small changes in the number of fastballs a hitter sees can help us project that hitter better. 

Sort the leaderboard for lowest fastball percentage, and you’ll see it immediately. It’s full of sluggers at the top. Reverse the filter and it’s mostly slappy speedsters. Rob Arthur took a more scientific approach and showed that isolated slugging and fastball percentage are indeed correlated negatively — sluggers see fewer fastballs

Rookies see more fastballs when they come into the league. Over the past five years, the league saw 57.5% fastballs, and rookies saw 58% fastballs. That’s not a large difference, but it comes in a large sample. Then again, it’s not a large difference, period. Over the course of a season, a rookie with 600 plate appearances would be expected to see 12 or so extra fastballs. 

In any case, even if this effect is small when you zoom out, it seems that individual differences in fastball percentage are predictive of future strong work. As Arthur said when he did the gory math behind this statement, "Fastball frequency normally varies according to the pop of the batter, so that when it changes, it may be indicating a change in the underlying skill level of the same batter."

And that makes sense — there are plenty of powerful rookies that might see fewer fastballs and throw off the league-wide numbers, but if you look at one player and see a dramatic difference in how he’s pitched, that means the league has changed their mind about a player. And therefore, you should, too. 

Before we make our list of players, it’s important to note that even Arthur had a strong caveat in his work. The more we know about a player’s major-league work, the less powerful a change in fastball percentage is for prediction. So, in order to really find some interesting breakouts, let’s limit our population to those with fewer than 800 career plate appearances. 

So, which young players saw fewer fastballs in the second half last season? 

Name 1st Half FB% 2nd Half FB% FB Diff
Rougned Odor 57.0% 49.6% 7.4%
Kolten Wong 59.0% 54.0% 5.0%
Corey Dickerson 62.7% 58.4% 4.3%
Nick Castellanos 53.2% 49.4% 3.8%
Travis d’Arnaud 59.3% 55.6% 3.7%
Scooter Gennett 58.9% 56.0% 2.9%
Marwin Gonzalez 57.6% 54.7% 2.9%
Mike Zunino 54.4% 51.9% 2.5%
Jon Singleton 58.3% 55.8% 2.5%
Yasmani Grandal 57.9% 55.5% 2.4%

In general, this list should get you giddy. Seven of these players were ranked on Baseball America’s top 100 list, and those seven averaged a peak of 34th on the list. On average, the group was around 23 years old last year, and is pre-peak by any measure. Maybe the best part about the list is that the players all have question marks — none is projected to be much more than an average player next year. Travis d’Arnaud’s 2.9-win projection by Steamer leads the group, and two wins is your traditional benchmark for an average player. 

Let’s take a look at each player in particular:

Rougned Odor

When Jurickson Profar went down with a shoulder problem, Odor took his equally interesting name and game to Arlington and had generally good results. A lack of patience hurt him, but with weighted offense that was only 10 percent worse than league average, he was better than most second basemen. Odor did hit nine home runs in about two-thirds of a season, but judged by isolated slugging percentage, he was about average (.142 ISO, .135 was average). Steamer projects him for the same sort of work next season (.139 ISO), but this metric suggests that pitchers were more afraid of him in the second half. Look at his Double-A performance (.197 combined ISO, league average was .136), and you see more power upside. Odor is also the youngest player on this list. Profar is slowly getting healthy, but Odor might be the guy you want to keep your nose pointed toward, as long as he can tame the whiffs.

Kolten Wong

Wong shouldn’t be a surprise on this list, at least not if you watched him destroy the postseason last year. Seven extra-base hits in 30 plate appearances is one way of saying it, big game-winning home runs is another way of saying it. With that big leg kick, maybe we should have known that he was looking to hit for power. Though Wong also had some better power numbers in the minor leagues (.164 Triple-A ISO, league average in PCL was .146), Steamer projects Wong to take a step back in power (.125 ISO). At 24 years old, with pitchers showing some reluctance to throw him fastballs — maybe Wong will take a step forward this year. 

Corey Dickerson

Judged only by their bats, Dickerson led this group. Perhaps he already had his breakout, since his weighted offense was 40 percent better than league average last year. There is one last place that more power could come from, though. Dickerson has, to date, hit more groundballs than flyballs. Hitters, as a population, hit fewer groundballs year-over-year until they turn 27 or 28. Between that possible growth, as well as maybe facing more lefties and leaving his platoon role behind, there is a chance that the 25-year-old Dickerson could do more in his encore. It’s not surprising pitchers threw him fewer fastballs as the season wore on. 

Nick Castellanos

Castellanos peaked as Baseball America’s 21st-best prospect after the 2012 season, and that’s good for second-best in this group. Scouts have always loved his hit tool, although that is something that has shown better in his batting average on balls in play in the minors than in his contact rates. Last season, Castellanos followed suit with below-average contact rates but good outcomes when he did make contact. The .326 BABIP was above average, and the .135 ISO was right on average. But despite weighted offense that was below league average, pitchers were more careful with him in the second half. Steamer likes the 22-year-old for a modest increase in power output (.144 projected ISO), but should we like his sweet swing more? 

Travis d’Arnaud

It’s not surprising that the old man of the group is also a catcher. Catchers debut later, on average, and it takes them a while to learn what is a tough position on both sides of the plate. In his second taste of the big leagues in 2014, d’Arnaud made more contact and actually already showed above-average power (.174 ISO). Don’t look too longingly at his Triple-A power stats (.289 ISO), though. He did amass almost a full season there over three years, but all three years were in Las Vegas, a hitter’s park in a hitter’s league. Still, a young catcher entering his peak years, and already putting some fear into pitchers? You could be looking at this year’s Devin Mesoraco. 

Scooter Gennett

A five-foot-10 middle infielder with this sort of nickname and these sorts of minor-league power numbers (combined .112 minor league ISO) isn’t supposed to strike fear into pitcher. Perhaps this name, and the next on the list, should remind us that, though Arthur’s work was impressive, it isn’t meant to stand alone. By itself, fastball difference only explained about 8 percent of the variance in the second year’s offensive production. You really want to put this in context. And in context, you might not want to bank on much power from a line-drive-hitting,  groundball-slapping middle infielder. Even if he’s been surprising at the major-league level so far. 

Marwin Gonzalez

You might throw fewer fastballs to a guy because he mashes fastballs. Or you might throw fewer fastballs to a guy because he can’t hit anything else. Judging by his pitch type values at FanGraphs, Gonzalez is more the latter than the former. For his career, he’s had negative outcomes on every non-fastball pitch. He’s, at best, the super utility guy for the Astros this year. It’s doubtful pitchers were afraid of throwing him fastballs.

Mike Zunino

What’s interesting about Zunino is that, by those pitch type values, he did bad work against fastballs last season. And yet pitchers threw him fewer fastballs. Then again, the one thing Zunino does well at the plate is hit for power, and his best power numbers come against hard stuff (.218 ISO against fastballs, career), so you might as well throw him fewer fastballs. Since he misses a third of the time and they’re giving him less of what he likes … it seems hard to predict a power breakout from the Mariners’ backstop. 

Jon Singleton

Kind of amazing to see Singleton on this list after his debut. Nobody struck out at a higher percentage than him last season, so if pitchers were throwing him fewer fastballs, it probably wasn’t from fear. Even though he had negative values on every type of pitch last season, Singleton was less bad at fastballs. So I guess they just figured they could throw him junk until he would adjust. Doubtful that his fastball percentage is a great harbinger of a breakout for him. 

Yasmani Grandal

Grandal shows up as a positive on so many lists that he absolutely has to break out this year. Grandal only peaked at 53rd on Baseball America’s prospect list, but he was a first-round pick who did have pedigree before some of the early stumbles we can expect out of a catcher. His minor-league stats suggest there’s some growth left, he hasn’t had good batted ball luck yet, and he just moved out of one of the worst parks for power in the big leagues. He’s already broken out a little bit, and got fewer fastballs for it. But it looks like there’s a little bit left still to go.

How many fastballs a pitcher throws a young hitter is only a small piece of the puzzle that is projecting a player with a limited track record. It can be helpful to look at how pitchers are attacking hitters, and how their game plans have changed, but it’s not the only prism through which we should be looking. 

But it certainly looks like pitchers are already treating Odor, Wong, Dickerson, d’Arnaud, and Grandal as bats worthy of fear.