Zac Gallen’s Stuff: A Follow-up (2024)

I first looked at Zac Gallen’s Stuff metrics after his first two games of the season and noticed that they were way off the numbers he posted in past years. I did expect his numbers to rebound closer to his career averages as the season progressed (and according to the poll in that first article 50% of you thought so, too), but there is fairly strong evidence to suggest that there was no way his grades would get back to the same neighborhood as his 2022/23 levels.

Stuff+ and BotStf claims to only require about 80 pitches to stabilize. To test that claim, I looked at the Stuff+ for pitchers who pitched at least 8 innings through April 9, 2023 (about 2 full turns through the rotation for most teams) and then compared that to the end of season numbers for all those pitchers who threw at least 120 innings. I was left with a pool of 64 pitchers, and of those 64, only six pitchers had a swing of 10 points or more in their Stuff+ from beginning to end of year. Additionally, only two of those six were improvements of at least 10 points while the other four were all regression of at least 10 points. If Zac Gallen is going to improve his Stuff+ grades to anywhere close to his 2022/23 numbers, he is going to have to be an extreme outlier.

After the Diamondbacks ace’s April 2nd start against the Yankees, Gallen’s Stuff grades were 86 (Stuff+) and 42 (BotStf). Let’s see where he’s at now:

Both Gallen’s Stuff+ and BotStf grades are higher than after his first two starts, but they are still off of his 2023 grades. In fact, every single one of his grades as measured by Stuff+ and PitchingBot are lower than at this point last year, and most of them are significantly lower. After 5 starts last year, Gallen’s grades barely moved a tick in any direction by the end of the season. Knowing this, I think we can safely assume that Gallen’s Stuff+ and PitchingBot numbers will only adjust tick or two more between now and the end of the year.

As it stands, his Stuff+ is graded about 10 points lower than last year, his Command as measured by PitchingBot is almost 20 points lower than last year, and his Pitching+ and BotOvr numbers will be off significantly as well. There are a couple questions we could ask about this. First, why is his stuff grading out lower? Second, and most importantly, does it matter?

Why is his Stuff grading out lower?

With respect to the first question, I wanted to take a look at how his pitches are moving in relation to previous years. Since Stuff+ and most movement metrics are compared to league average, maybe Gallen is throwing the same as always, but the league is changing around him. Below is a screen grab from Baseball Savant which gives his pitch movement in raw numbers as well as in relation to league average. I chose to present Baseball Savant’s table because of the color coding that shows the stark difference in this year’s movement and last year’s.

baseballsavant.mlb.com

The first thing that jumps out is how Gallen has had well above average vertical movement on his pitches in 2022/23, but this year, he is generating no positive vertical movement in relation to league average. The second thing that I noticed as I looked at the vertical movement was that nearly all of his pitches are dropping more than last year, but are still being considered below average. That quirk can be explained by how Baseball Savant presents it’s movement data. At the top of the pitch movement stats, this blurb explains how more movement can be less than average:

“Every pitch is affected by the forces of gravity, which means that every pitch drops on its way from the mound to the plate. These numbers are reported with gravity, which makes them larger and different than other pitch movement numbers you may have seen. Since gravity requires time, and slower pitches aren’t ‘better’ just because they have more time to move, the movement of a pitch is compared to ‘average’ movement by comparing it to other MLB pitch types within +/- 2 MPH and from within +/- 0.5 feet of extension and release.”

Basically, that means that Gallen’s extra 2.5” of downward movement on his knuckle-curve are not as impressive when it is coming in over 2mph slower than last season. When taking Gallen’s lesser velocity into account, that extra downward movement is better explained by gravity than anything Gallen is doing. Gallen has also added more horizontal movement compared to last year’s pitches, but, according to Baseball Savant’s percentile rankings based on velocity, this extra horizontal movement is effectively the same as 2023 in relation to league average. According to the formulas, the extra movement Gallen’s added has been effectively negated by the lower velocity.

Speaking of Gallen’s velocity, on average he is clocking in about 1-2mph behind his velocities last year, although his fastball may be coming back around to 2023 levels. Gallen’s average fastball velocity this year is 92.5mph, but his most recent start against the Giants he averaged 93.5mph, identical to his 2023 average fastball velocity. His knuckle-curve and slider were both still coming in well below his 2023 average velocities, however.

I think the final answer to the first question of why Gallen’s Stuff is being graded lower is that his velocity is just a tick or two off from past years, and that drop in velocity hasn’t been counteracted by a meaningful jump in movement.

Does it matter?

The second question that I asked about Zac Gallen’s Stuff metrics is probably the most relevant to everyone reading: does the drop in Stuff matter? After all, if the Milkman is still able to retire batters, does it really matter how much his four-seamer is moving in relation to the average?

To answer this question, I looked back at the last two seasons to see how qualifying pitchers performed when they had Stuff+ grades 10 points lower than the previous year, which is what Gallen is on pace to do this year. In 2023, there were 20 pitchers who qualified in both 2022 and 2023. Of those 20, only 4 had Stuff+ grades that were 10 points lower than their previous season. Those four were Sandy Alcantara, Justin Verlander, Dylan Cease, and Gerrit Cole. An interesting mix of pitchers there, with the American League Cy Young award winner accompanied by three pitchers who fell short of their results from the previous year. There were also only 4 qualifying pitchers who had Stuff+ drops of greater than 10 points from 2021 to 2022. The pitchers in that bucket were Yu Darvish, Julio Urias, Joe Musgrove, and Jose Berrios. Of those four, three actually had lower ERAs when compared to their previous year (Berrios is the lone under-performer). An even split of pitchers who regressed or impressed based on their previous year’s results does little to let us know whether it is a bad thing to grade out lower in Stuff+. In fact, of the eight names above, three earned Cy Young votes after having “lesser” stuff than the prior year. Really, the only red flag that can possibly be raised when it comes to looking at those eight pitchers is that there is only one who went from a Stuff+ score of above 100 to below, and that was Berrios in 2022 when his lower Stuff+ grade was accompanied by his ERA rising nearly 2 runs to 5.23 and his Hard Hit rate jumped up 5% compared to his previous season. Gallen is similarly on pace to go from a Stuff+ above 100 to below and his Hard Hit rate is currently up 4.5% on last year’s already high rate.

So does this significant drop in Stuff matter? I can’t say that it does. At least not by looking at previous years’ data. We can’t complain about Gallen’s real world results thus far, that’s for sure. His 3.00 ERA is backed up by his ERA estimators. His K% is the highest it’s been since 2019 and 2020 and he’s generating more Whiffs than the last couple years, as well. Conversely, he’s throwing more pitches outside the zone than ever before which is leading to more walks and, when he does find the zone, he’s getting hit hard more often than ever in his career.

As long as he can continue to effectively pitch outside the zone and generate whiffs, this season could go down as one of the seasons where a Stuff+ ‘faller’ earns Cy Young votes. Honestly, I see no reason why Gallen can’t be one of those overachievers because he seems to excel in all the areas that can’t be measured by Statcast and FanGraphs. He knows how to pitch, plain and simple, and pitching is way more than spin rates and velocity.