Last year around this time, Diamondbacks right-hander Zac Gallen appeared to be closing in on his first NL Cy Young Award.

After a dominant outing against the Texas Rangers on Aug. 22, 2023, Gallen was 14-5 with a 3.11 ERA, 1.06 WHIP and 179 strikeouts to 34 walks in 168 innings.

Some National League pitchers could match his innings total. Others had equal or even better run prevention numbers. But no one was particularly close to matching both. Gallen was the clear frontrunner.

But down the stretch, Gallen struggled. In his final seven starts of the season, he posted a 4.93 ERA. In that same timeframe, then-San Diego Padres lefty Blake Snell logged a ridiculous 0.47 ERA in six starts. Gallen wound up finishing third in Cy Young voting behind Snell and San Francisco Giants right-hander Logan Webb.

While narrowly missing out on a Cy Young is hardly cause for embarrassment, Gallen has not been the same since last August.

Over his past 34 starts, including the playoffs, Gallen has a 4.22 ERA, 1.33 WHIP and 167 strikeouts to 64 walks in 185 2/3 innings. Those are perfectly fine numbers for a mid-rotation starter. But they are hardly indicative of the ace-caliber pitcher that Gallen has been for much of his career.

On Saturday in Tampa Bay, Gallen turned in his latest subpar performance, allowing four runs on nine hits over five innings against the Rays. The outing brought Gallen’s season ERA to 3.85.

After missing a month with a hamstring injury from late May to late June, Gallen tossed six scoreless, one-hit innings on June 29. Since then, his nine starts have been somewhat disastrous: 5.24 ERA, 1.66 WHIP and 35 strikeouts to 21 walks in 46 1/3 innings.

“I feel like I’ve just been grinding, really,” he told reporters, including the Arizona Republic, after Saturday’s game. “It’s always back to the drawing board. One of these days, it’s going to click.”

Granted, Gallen probably deserved better than what his final line showed on Saturday.

In the first inning, he was charged with a run because a Rays baserunner stole home as part of a double steal. In the fourth inning, what initially appeared to be an inning-ending double play was ruled an infield single because the ball grazed an umpire on its way to second base. In the fifth, a seemingly catchable fly ball was scored a double because it got stuck in the bizarre ceiling apparatus at Tropicana Field.

Nonetheless, Gallen is currently mired in one of the worst stretches of his career, and Saturday’s outing felt like more of the same.

So, what’s going on with Gallen over these past couple months? What about the past year as a whole?

There are certainly more factors than what will be covered here, but one of them seems pretty clear: the mysterious regression of Gallen’s once-dominant four-seam fastball.

What happened to Zac Gallen’s fastball?

Even when Gallen was at his best in 2023, he often spoke about being unsatisfied with the feel of his pitches.

For example, on May 8, 2023, Gallen tossed seven innings of one-run ball against the Miami Marlins. Then, he told reporters after the game that he “didn’t have much semblance of an off-speed pitch.”

While it would be easy to write off Gallen’s comment as perfectionist talk, that May 8 start is a helpful case study.

Gallen leaned heavily on his four-seam fastball that day, throwing it 60 percent of the time. Of the 11 whiffs that he generated, eight came on the fastball. The heater was also the source of 14 of his 17 called strikes.

The closer you look, the more you realize: Gallen’s secondary stuff really was off that day. But his four-seam fastball was so good that it basically didn’t matter.

That storyline was not just a one-off for Gallen. He has had one of the better four-seamers in baseball for a long time. It’s been an anchor, even as his feel for his other weapons has come and gone.

Based on Baseball Savant’s run value, Gallen’s four-seam fastball was tied for the third-most valuable pitch in all of baseball last year, trailing only New York Yankees ace Gerrit Cole’s four-seamer and Logan Webb’s changeup. Every year since Gallen broke into the majors in 2019, run value has pegged Gallen’s four-seamer as an above-average weapon — that is, until this year.

Consider how Gallen’s four-seamer has fared in 2024 compared to the previous two seasons:

So far this year, Gallen’s fastball is getting hit harder and generating fewer whiffs than ever before.

Naturally, this leads to the question of whether the velocity or movement on Gallen’s fastball have changed. Here’s a look:

From a stuff standpoint, Gallen’s fastball was arguably at its best in 2022, when it averaged a career-high 94.1 mph with a stellar 17.6 inches of induced vertical break. Note that 15 inches of IVB is around average, and 18 inches is considered excellent. The higher the IVB, the more the ball will look like it’s rising to the hitter, which generally makes it harder to square up.

In 2023, Gallen’s average fastball IVB dropped to 16.9 inches, and it has remained there in 2024. Gallen’s average fastball velocity has seen effectively no change year-over-year, although his velocity readings have fluctuated more. Since the start of July, Gallen has thrown both the hardest fastball of his career (97.9 mph) and the slowest fastball of his D-backs career (88.8 mph). The latter happened in his Saturday start against the Rays.

According to Fangraphs’ Stuff+ metric, a pitch grading model that is based on physical characteristics (movement, velocity, release point, etc.), Gallen’s four-seamer went from a well-above-average offering in 2022 (116 Stuff+) to slightly above-average in 2023 (104 Stuff+) to now just average in 2024 (100 Stuff+).

All that is to say: From a stuff standpoint, this is theoretically the worst that Gallen’s fastball has ever been.

At the same time, the delta from last year to this year appears to be relatively small. Given that Gallen’s fastball in 2023 was among the best pitches in the majors, there’s probably something else going on here.

And that brings us to a second, likely more important topic: command.

Zac Gallen’s diminished fastball command

So far in 2024, Gallen has a 7.9 percent walk rate. While that is still better than the league average, it is also Gallen’s highest mark since 2021.

Ironically, Gallen has career-high strike rates this year on both his changeup and knuckle curve, in part because both pitches are generating chase at career-best clips.

By all accounts, Gallen’s uptick in walks primarily traces back to his fastball. So far in 2024, 60.9 percent of Gallen’s four-seamers have gone for strikes. Compare that to 69.2 percent a year ago. The league average strike rate for four-seamers is 66.3 percent.

Gallen’s first-pitch strike rate has also decreased dramatically. It’s 56 percent so far in 2024, down from 66.1 percent last year.

The difference between 0-1 and 1-0 on a hitter is probably bigger than you think. This year, opposing batters have a .588 OPS against Gallen in at-bats that start with an 0-1 count. They have an .822 OPS in at-bats that start 1-0.

While first-pitch strike issues have clearly played a significant role in Gallen’s struggles dating back to last August, Gallen threw first-pitch strikes to a solid 64 percent of Rays hitters on Saturday. He did not struggle with his first pitch to Rays hitters, but he did struggle to put them away.

Of the 98 pitches that Gallen threw on Saturday, 34 came with two strikes. It was his third-highest total of two-strike pitches in any start this year.

So, why did Gallen have so much trouble finishing Rays hitters off? Here is a look at all of those two-strike pitches:

While Gallen’s changeup and knuckle curve locations look pretty good — Rays hitters did well to waste or lay off many of them — his fastball location was subpar.

Of the 12 two-strike four-seamers that Gallen threw on Saturday, only two resulted in an out: a whiff by Rays infielder Christopher Morel for strike three and a deep fly out by rookie slugger Junior Caminero. Five of those 12 four-seamers were taken for balls, and one was knocked for a two-run single. With two strikes, those are poor results.

Much like Gallen’s downtick in first-pitch strikes, inconsistent execution on two-strike heaters has emerged as a trend.

From the start of the year through July 1, Gallen generated a strikeout on 22.5 percent of his two-strike four-seamers. Since July 1 — in this dreaded span of nine starts in which he has a 5.24 ERA — Gallen has generated a strikeout on just 11.4 percent of his two-strike four-seamers.

Again, location is a big part of this. Here is a look at every two-strike four-seamer that Gallen has thrown since July 1:

Just 37.5 percent of these four-seamers are in the zone, and many of them are big misses; the types of pitches that major leaguers will spit on without batting an eye.

Struggling to get chase on four-seamers has been a trend at large for Gallen. Last year, Gallen posted a 25.1 percent chase rate on his four-seam fastball. This year, that figure has dipped to just 15.6 percent, well below the league average of 23.9 percent.

Put all this together, and it makes sense why Gallen is issuing more walks and striking out fewer batters in 2024. Gallen’s four-seam fastball is his most-used pitch by far, and not only is he throwing fewer of them in the strike zone than ever before, but he is also getting significantly less chase on those that are out of the zone.

This hypothesis that Gallen’s fastball command has taken a significant hit this year is confirmed by another Fangraphs metric: Location+, which measures quality of pitch location with 100 being average. Last year, Gallen’s four-seam fastball Location+ was 104. This year, it is just 94.

Not only is that by far the worst mark of Gallen’s career — Fangraphs’ Location+ data goes back to 2020 and Gallen’s has been 101 or higher every year — but it is also among the worst fastball Location+ marks on the Diamondbacks in 2024.

Among Diamondbacks pitchers who have thrown 100 or more four-seam fastballs in 2024, only rookie right-hander Yilber Diaz has a lower Location+ on his fastball than Gallen. Diaz’s comes in at 93.

Of course, it’s one thing to say that Gallen’s fastball command is off. It’s a whole other thing to explain why that is. The latter question is a more challenging one. Based on Gallen’s postgame comments on Saturday, he might not be too sure himself, at least not yet.

Given Gallen’s long track record of success — and the fact that, from a pure stuff standpoint, his current four-seamer is not all that dissimilar from the one that graded out as one of the best in baseball a year ago — it seems possible that the once-elite Gallen fastball will return before long.

For now, it helps a lot that, even after getting swept by the Rays over the weekend, the Diamondbacks have a 4 1/2-game cushion on the New York Mets in the National League wild card race. There is considerably less pressure on Gallen than there was a year ago, when the team was trying to claw its way back into the playoff picture after a historic slump.

But even if the Diamondbacks can make the playoffs without Gallen at his best, they’d need him in a big way once they got there. Last year, the Diamondbacks made it all the way to the World Series despite losing four of six games started by their ace. That probably won’t happen again.

Follow Jesse Friedman on Twitter

Top photo: Kim Klement Neitzel/USA TODAY Sports