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Three things I’ll be watching in George Kirby’s start tonight

What’s the next step for the Westchester Wizard?

MLB: Tampa Bay Rays at Seattle Mariners Joe Nicholson-USA TODAY Sports

Every scouting report on George Kirby starts with his plus-plus command. And indeed, we’ve seen him show that off in his first 20 innings in MLB, as he’s walked just two batters and only gotten to five 3-0 counts. But command alone won’t dominate the best hitters in the world. So here are three things I’ll be watching for as the Westchester Wizard looks to take the next step.

1. Is he willing to commit to his fastball?

At first glance, this might seem counter to modern pitching advice, as fastball rates continue to plummet across the league:

Underlying data via Fangraphs

But that’s because for most pitchers, a breaking ball is their best pitch. For George Kirby, though, nothing compares to his four-seamer. He’s generating swings and misses on it 17.9% of the time, almost double the league average of 9.8% among starters. 161 starters have thrown at least 50 four-seamers this year, and only Eric Lauer has a higher swinging strike rate. Soak that in. Gerrit Cole, Zack Wheeler, and Brandon Woodruff, all aces with famous four-seamers, are getting the best result less often than Seattle’s rookie. And yet, Kirby’s willingness to go to the pitch has varied quite a bit.

In his two outings where he threw his four-seamer 60-ish% of the time, he dominated, striking out a combined 16 while issuing no free passes over 11 innings. By contrast, when he dipped under 50% against the Mets and Red Sox, he got beat up and only had four strikeouts to two walks over nine combined innings.

To be sure, pitchers need to throw multiple pitches (unless they’re Mariano Rivera), and to get through the order more than once, almost everyone needs a third pitch, too. Moreover, you can’t develop your secondaries unless you’re willing to throw them. But George Kirby’s biggest job right now is to gain confidence, so for the time being, I’m hoping he leans on his best pitch.

2. Does he have the feel for his changeup?

For now, Kirby’s most comfortable with his slider among his secondaries. In his first outing, he didn’t throw anything but fastballs and sliders until his second time through the order. But going forward, I think the more important pitch will be his changeup. When he’s got it, it’s a thing of beauty. Here he is using it to fool contact hitter Brandon Nimmo:

It looks a lot like his fastball: it has a similar spin axis and is delivered with similar arm speed. But it’s 10 mph slower and drops more than twice as much on the way to the plate. If a hitter has somehow timed up the fastball, they’re liable to swing right over the change. For that to work, though, Kirby’s needs to have command of the pitch that day. It’s famously a feel pitch, so it can come and go. Here’s what it looks like when he’s got it, as he did against the Mets:

That’s not a great swing-and-miss pitch yet, but they’re tightly clustered near the arm-side bottom corner. It was tempting enough to get swings, and when hitters connected, they didn’t do much with it. By contrast, here’s what it looks like when he doesn’t have it, like against the Red Sox, when he at least knew not to throw it that much:

Those pitches aren’t nearly as competitive and include one of Trevor Story’s home runs. Through his first four outings, Kirby has run an xwOBA of .536 on his changeup, which is basically prime Barry Bonds. When he had it against the Mets, though, the only changeups hit into play were converted into outs (and one error). Once he’s able to command it more regularly, hitters will have to keep it in mind as a serious threat. That’ll not only make a good pitch on its own, it’ll also help his fastball play even better, as hitters get caught in between, unable to commit to the 95 mph pitch or the 85 mph one.

3. Can he get even a small boost in his ground-ball rate?

Through his tour of the minor leagues, Kirby ran a ground-ball rate over 50%. But through his first four starts in the bigs, he’s only keeping the ball on the ground 45.2% of the time. Groundballs lead to lower ERAs because although more groundballs become hits, far fewer go for extra bases. When a pitcher can limit the traffic by reducing walks, like, say, George Kirby can, groundballs barely become a threat at all. The necessary consequence of the lower ground-ball rate is higher rates of line drives and fly balls, which are dragging down his overall numbers.

Contact-type rates start to stabilize around 80 batters faced and aren’t reliable until closer to 200. Kirby’s at 85, so it’s not a big enough sample to be concerning, but it is big enough to be keeping an eye on.

The question of whether Kirby gets some positive regression or has a new normal comes with pretty high stakes. Through his first four starts, he’s running an xFIP of 3.12. That’s already outstanding—26th out of 151 starters with at least 20 innings pitched. But I recalculated his xFIP using his minor league contact-type rates and got 2.70. That’d move him up to eighth in baseball. We should manage our expectations, of course. It’s not as simple as generate more grounders, become a top-ten pitcher, profit. But the magnitude of the difference between 2.70/8th and 3.12/26th illustrates that even a small change matters quite a bit.

Traditionally, the changeup is the go-to pitch when someone needs a grounder, as the extra drop gets hitters on top of it. And even though Kirby’s only throwing changeups 12.7% of the time, they account for 18.5% of his groundballs so far. So there’s all the more reason to develop his change. But there’s not one weird trick to getting groundballs; he’s also going to have to keep his slider and curve on the bottom edge of the zone as well as continue to locate his fastball.

Bonus fourth thing I’m watching: Can he make Adley look bad..ley?

Here’s a bonus thing to watch for those who are more into the narrative than the analytics. Heading into the year, the only team with a better set of top prospects than Seattle was Baltimore. They held the elite one-two punch of the highest ranked position player (Adley Rutschman) and the highest ranked pitcher (Grayson Rodriguez). Seattle, meanwhile, had the third highest position player (Julio) and fourth highest pitcher (Kirby). So it’d be extra sweet if the Mariners’ top pitching prospect could shut down the Orioles’ best position-player prospect, who got the call last week. These are the plate appearances I’ll be watching most closely.