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Zac Gallen Has Some Intriguing Upside

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The latest Zac(k) G to join the team could have a very bright future.

Miami Marlins Photo Day Photo by Streeter Lecka/Getty Images

The Diamondbacks really have a thing for Zac(k) G’s. After starting the year with two of them - Zack Godley and Zack Greinke - the team decided to pick up young right-hander Zac Gallen from the Miami Marlins in exchange for the risky-but-toolsy Jazz Chisholm. Following this acquisition, the Dbacks apparently had found “their guy” and promptly traded Zack Greinke to the Houston Astros and DFA’d Zack Godley. Suddenly, the Diamondbacks are down to one Zac(k) G: Zac Gallen.

This trade came as quite a surprise to many Dbacks fans - this author included - but it appears that the Diamondbacks have stumbled upon a possibly really good starting pitcher. Gallen wasn’t an overly hyped prospect prior to this year, but he made some changes and improved the velo on his fastball. Suddenly, he was absolutely dominating the PCL:

Zac Gallen in the PCL: 1.77 ERA

Second place in the PCL: 3.65 ERA

Now this is among qualified starting pitchers but that is a huge difference. Gallen had the luxury of not playing in the Pacific Division of the PCL for most of his games, but there was no other pitcher from his division that was dominating like him, either. And it wasn’t just ERA - his FIP was 1.63 points better than second place and his 33.6% K% blew away everyone else, with Héctor Noesi in second at 26.1% and fellow Diamondback Taylor Widener in third at 23.4%. Gallen was absolutely dominating the PCL and no other starting pitcher was remotely close to him.

This, so far, translated into MLB success for Gallen - 7 GS, 36.1 IP, 2.72 ERA, 3.58 FIP, 28.5% K%, 11.9% BB%, and 0.74 HR/9. The walks have been a bit of an issue for Gallen but he has so far shown an excellent ability to generate strikeouts. And so far, it’s not a mirage - his 28.8% K% suggests he could be doing slightly better.

Gallen primarily throws four pitches - four-seam fastball, changeup, cutter, and curveball - but his best two pitches have been his fastball and his changeup. And so far, they have been really good. Here are how his pitches rate per FanGraphs (this metric is runs above average per 100 pitches):

Fastball: 2.02 wFB/C (2nd out of 181 pitchers)

Changeup: 4.01 wCH/C (3rd out of 169)

Cutter: 0.03 wCT/C (27th out of 63)

Curveball: -2.44 wCB/C (126th out 154)

His fastball and changeup have been utterly dominant in his brief MLB stint and his cutter has been solid. His curveball, so far, rates poorly. Keep in mind, that these stats are very results-driven (e.g. batted ball; more will be discussed below) and the samples are small. But it helps to paint the picture of who Zac Gallen is. Let’s go through his arsenal.

Fastball and Changeup

As his bread and butter, these two pitches will be lumped together as they are performing very well and have a lot of strong underlying support. Even with the “improved” velocity as noted above, Gallen’s average fastball velo is still pretty low, coming in at the 18th percentile per Statcast. However, he has gotten as high as about 96.5 MPH so he is definitely capable of getting it up there when needed.

Zac Gallen Fastball and Changeup Percentile Rankings

Metric Fastball Changeup
Metric Fastball Changeup
Whiff% 73 96
xwOBA 84 88
Exit Velo 89 63

Velocity aside, the other attributes of his fastball are pretty crazy. His fastball sits in the 73rd percentile for spin which generally means he is able to generate a lot of swings and misses with the pitch - which is very true, considering his whiff rate on fastballs is also right at the 73rd percentile. Pretty convenient. But also check out that batted ball profile: the xwOBA is up there at the 84th percentile and his average exit velocity sits in the 89th. In addition to a ton of whiffs, he really limits good contact on it - so far this season, he has yet to give up an extra base hit on his fastballs. In 28 batted balls, hitters have 6 singles and that’s it. Impressive.

But there’s more: his fastball is very flyball-oriented but it also generates both a ton of foul balls and pop ups. Out of 197 pitchers with at least 100 four-seam fastballs thrown this season, he ranks 12th in fouls/swing (50.94%) and 11th in popup% (18%). Combine that high foul rate with his already high whiff%, that means very few of the fastballs that are swung at ever make it into play. And that’s a good thing - you can’t score runs if you can’t hit it in to play.

Combined, only 22.7% of pitches swung at against Gallen make it into play. This is exceptionally low - out of 117 pitchers, he would rank 9th. And take a look at the guys ahead of him:

  1. Gerrit Cole (20.4%)
  2. Chris Sale (21.1%)
  3. Blake Snell (21.3%)
  4. Max Scherzer (21.3%)
  5. Justin Verlander (21.5%)
  6. Jacob DeGrom (22.1%)
  7. Caleb Smith (22.2%)
  8. Robbie Ray (22.5%).

That is some massively impressive company to be around, isn’t it? Despite the relatively low velo, Zac Gallen has a GOOD fastball. That is certainly a good thing to have as a young pitcher.

But yet, there’s still that changeup. Did you see that whiff rate sitting in the 96th percentile? The changeup actually gets hit a bit harder than his fastball, relatively speaking, but the insane improvement in whiff rate has made it out to be a better pitch so far. In fact, he has more strikeouts with the pitch (10) than he has balls into play (9). And of those 9 balls in play, 6 of them were grounders. That 67% GB% is the 12th-highest out of 158 pitchers but considering it’s only a sample of 9 batted balls, it’s too early to tell if that’s real or not.

What’s really surprising about his changeup is that it’s his least-thrown pitch of the arsenal. Maybe that helps add to its effect but if it’s truly this good, it might be worth throwing more. It’ll be interesting to see if the team tries to change some things with Gallen.

It also appears that there might be some really good tunnel effect on his pitches, though the data is not yet available on Baseball Prospectus to confirm. In the meantime, enjoy these awesome GIFs:

Curveball and Cutter

With his fastball and changeup as his bread and butter, we’ve established where Gallen’s current strengths are. Now it’s time to assess the potential of his other two offerings.

His curveball has been his “worst” pitch so far, but that’s primarily due to the fact that he’s given up three homers with it (and no homers on any other pitch). It actually ranks pretty decently on Statcast with a solid .303 xwOBA but there is nothing really spectacular with the pitch as it stands currently. He generates a lot swings on it - 12th-highest out of 155 pitchers - but everything else is very middle of the pack: spin rate, whiff%, and fouls/swing. The exit velo has been a little above average - 66th percentile - but that’s about it. Without another improvement of some sort, it’s hard to see this being a real standout pitch for Gallen and with the high swing rate, batters are obviously able to identify it out of the hand.

The real potential upside for Gallen comes from his cutter. He has the 7th-most drop on the pitch among 125 pitchers at 35.3 inches but he also gets 7.6 inches of horizontal movement, which is the 2nd-best among the same group. That’s 215% more break than average. Let’s take a look at a sweet cutter (or slider according to Twitter):

This is where things get a little funky, as Statcast and Fangraphs view this as a cutter and Baseball Prospectus and PitchF/X treat it as a slider. Either way, the pitch is generating a ton of whiffs and is in the upper echelons of pitchers.

So far, the cutter hasn’t performed exceptionally well, as noted by the very average 0.03 wCT/C and it’s his current worse pitch per Statcast, with a .320 xwOBA. This is the pitch that will make Gallen breakout, however, if he can fine-tune it and get better results at the MLB level.


The Diamondbacks have happened upon themselves a really excellent young pitcher, complete with a full 6 years of team control. As he stands right now, Gallen really looks to be a good mid-rotation starting pitcher with his excellent fastball and power changeup. He has shown that he can strikeout major league hitters and he had excellent command and home run suppression in the minors. He’s struggled a bit with walks in the MLB but his scouting report and minor league track record suggest he can be better in this regard. If he can turn his cutter and/or curveball into another above-average pitch, Zac Gallen might start flirting with top-of-the-rotation considerations.

If you look at the “ace” criteria, Gallen meets four, maybe five, of the five requirements:

  • Top 100 Prospect - Yes
  • Above-average strikeouts in the minors - Yes (9.0 K/9 total, 11.0 K/9 this season)
  • Walk rates near league average - Yes (2.4 BB/9 career MiLB)
  • Home run suppression - Yes (0.8 HR/9 career MiLB)
  • Drafted early: Maybe (end of third round)

This is an exciting time for Diamondbacks fans. Even though the team lost Zack Greinke, there are a lot of young, exciting arms that we will be seeing more and more of over the next few years.