The Jake Arrieta signing by the numbers

In 2017 I watched Jake Arrieta throw what we all thought would be his last game as a Cub and I wrote this piece where I talked about Jake’s role with the Cubs as “the Stopper.” The pitcher who just always seemed to come through at big moments when the team needed him most. That piece contains a thought exercise where I made this argument:

The thing I think we miss about Jake sometimes, however, is his greatness for the Chicago Cubs, and I’m not talking about the stint in 2015 where he was the best pitcher in the history of the game (I’m sure most of the people reading this know I’m not being hyperbolic there, but if you don’t, read this). No, see, Jake was great even if you revert some of his 2015 numbers to the mean. Even if you eliminate the six month period where no one could hit against him. Even if you vacated his Cy Young Award.

Now, don’t get me wrong, I’m not saying we should ignore any of those things, but over the last three years I frequently got frustrated talking about Jake with Cubs fans. People seemed to think if he wasn’t an ace or the next Bob Gibson then he wasn’t fulfilling his role. Frankly, nothing could be further from the truth.

Jake Arrieta agreed to a $6 million, one-year contract with the Chicago Cubs Friday (not yet officially announced by the team) and will return to the North Side of Chicago. No one expects him to be 2015 Jake Arrieta, but honestly, no one thought he was 2015 Jake Arrieta when he came to Chicago in the first place. I think the Cubs thought they had a No. 3 starter who could bolster a rotation led by Jon Lester and then Jake Arrieta exceeded everyone’s expectations.

In 2021 the Cubs need him to bolster a rotation led by Kyle Hendricks, so let’s take a look at his numbers since 2017 to see what Jake can do for the Cubs this season:

A few things jump out in these numbers. Jake’s K/9 rate has steadily ticked down each year since he left the Cubs (I’ll have more on that below). Simultaneously, his walk rate has increased as has the number of home runs he gave up. As for that last number, I wanted to know how much of that is potentially the ball so below is a chart of the 27 home runs Arrieta gave up in 2019 and 2020 overlaid on a map of Wrigley Field. It does look like at least one home run was a park effect and at two others would not have made it out if the ball went back to 2017 levels:

Baseball Savant

The next thing I wanted to know is what was behind the K and BB rates and it looks like both went up because of a lower whiff rate on Arrieta’s sinker. When Jake was otherworldly in 2015 and 2016 the whiff rate on that pitch was 14.8 and 18 percent respectively. In Jake’s final year with the Cubs that whiff rate dropped slightly to 14.5 percent but it has steadily declined since then, in 2018 it was 13.2, in 2019 it was 11.3, and by 2020 it had absolutely cratered to 7.9 percent. As that whiff rate cratered Arrieta threw that pitch slightly less often, but he still threw it much more than he did in his most effective seasons with the Cubs as you can see below:

Baseball Savant

Over at Cubs Insider, Brendan Miller has a potentially novel fix for Jake Arrieta based on data from Kyle Hendricks over the last few seasons. I’d commend you to read the whole thing as it contains a lot of charts and information on ways Arrieta could be a more effective pitcher in Chicago than with the Phillies, but this part about “elevated sinkers” really jumped out at me:

Part of it is velocity, as Arrieta is throwing about 1 mph slower in his older age, but I don’t think that tells the whole story. A bigger culprit might be that Arrieta’s repertoire is stuck in the past when baseball was a completely different game. The game now features more swings built to generate launch angle against low pitches, which has been terrible for Arrieta’s heavy-sinker repertoire.

But do you know who has adapted to the launch angle revolution? The Cubs’ new pitching development team headed by Craig Breslow and implemented by pitching coach Tommy Hottovy.

The Cubs have encouraged their pitchers to elevate sinkers, a trend best visualized by the distribution graph below. What you will notice is that Kyle Hendricks (red) throws more high sinkers (further to the right on the figure) than Arrieta (blue) and MLB pitchers as a whole (grey). You’ll also notice that Arrieta tends to throw more sinkers lower in the zone than his counterparts.

Brendan Miller - Cubs Insider

There is one other element to keep an eye on as Arrieta returns to the Cubs and that is whether the defense playing behind him can help him over perform his FIP again at Wrigley Field.

We often think of Kyle Hendricks as the Cubs’ ground ball specialist but Jake Arrieta actually has a higher ground ball percentage than Hendricks over the last four seasons. Jake Arrieta has the 18th highest ground ball percentage among qualified pitchers in MLB over the last four years at 49.5 percent. By comparison Hendricks ranks 45th among qualified MLB pitchers at 46 percent. Could a return to the Cubs help Jake beat his FIP and xFIP again like Hendricks does year after year?

I have long thought there are two main reasons Hendricks overperforms his FIP and xFIP. The first is that he’s a command specialist and his ability to locate pitches outperforms the inputs that go into those metrics, however the second (and less discussed item) is that FIP and xFIP don’t take defense into account by design. Hendricks isn’t the only Cubs pitcher outperforming those numbers year in and year out — every qualified Cubs pitcher since 2016 has outperformed their FIP and xFIP except Jose Quintana. That signals to me that the Cubs defense was a factor in limiting runs and Arrieta, with that exceptional ability to generate ground balls, will benefit from a return to Chicago.

Takeaways and Intangibles

Anyone who thinks Jake will magically transform into his 2015 self when he returns to the North Side of Chicago is likely to be sorely disappointed, but the Cubs really don’t need him to do that. They would be perfectly happy with Jake overperforming his peripherals like he did in 2017 and there are a decent number of data points that indicate he can do just that with the Cubs. Jake provides a much more reliable number two or three option behind Kyle Hendricks and mixes up the very similar looks Hendricks, Alec Mills and Zach Davies will give batters.

Beyond the numbers, Jake has always given the Cubs some of the grittiest starts when the team needed it most. One of the biggest losses with failing to resign Jon Lester was the clubhouse presence Lester provided the rotation. Arrieta brings a different brand of that leadership to a young pitching staff that will benefit from it. And maybe, with a little luck, the Cubs will reach the postseason and we can see if Arrieta is still the stopper he’s always been in October.