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Projecting Miles Mikolas Using HR/FB Rates

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What happened to Mikolas between 2018-19? What does that mean for his projections in his return to the rotation?

St Louis Cardinals Photo Day Photo by Michael Reaves/Getty Images

Miles Mikolas is set to return to the Cardinals today! That makes this a great time to “bump” this article – the content anyway. I wrote about what changed between Mikolas’ 2018 and 2019 seasons and what to expect for 2021 – “Fixing Mikolas’ HR Rate”. Then the news broke (literally as I was finishing the article) that the club was shutting him down with shoulder pain. I adapted, posted the article anyway, and then the site promptly crashed for the rest of the morning.

So, pretty much my mom and I were the only ones who read the original piece. A few of you opened it – thanks for that – took one look at the title and then concluded that imminent Tommy John or shoulder surgery would take care of Mikolas’ homerun problem.

Thankfully what so many pessimists predicted hasn’t come true. (Yet.) Mikolas will start against the Cubs today after rehabbing with Memphis. Expect him to be on a controlled pitch count for a few weeks and for his results to be mixed. He hasn’t thrown a competitive MLB inning since late 2019. He’s going to be rusty, which is a bad trait in an extreme control pitcher.

When (or if) he can get himself right, though, the Cardinals will have a big boost in their rotation. Mikolas is a player who fits perfectly into the team’s philosophy of defense-first and pitch-to-the-edges for weak contact. He should be able to take advantage of Busch’s power suppressing tendencies.

That’s the substance of the argument in the original post, some of which is presented below. I’ll edit it quite a bit and add the relevant data for the first quarter of 2021, which is very encouraging – not only for Mikolas’ projections but the entire starting rotation. Then we’ll make our conclusions for Mikolas’ remaining season.

In 2019, Miles Mikolas was legitimately one of the best starters in the National League, producing a 4.2 fWAR and a 2.83 ERA in just over 200 innings pitched. He barely walked anyone. While his K numbers were surprisingly low considering his heavy fastball and quality slider, he generated ground balls at a high rate and kept the ball in spacious Busch as well as anyone.

Reasonable fans expected Mikolas to take a step backward in 2019 and he did. His walk rate stayed excellent. He improved his K rates. Harder overall contact led to a hefty +1.33 rise in ERA – 2.83 to 4.16 – and a +1.01 rise in FIP. His fWAR fell to 2.5. All ofthat was still solid but less than the team desired.

What caused the massive increase in runs allowed? Consider the following set of indicators. These are the kinds of things I would normally look at when evaluating a pitcher like Mikolas:

BB rate – increased from 3.6 to 4.2%. That’s a change of just .6 percentage points or three walks over a full season. There’s nothing here.

K rate – increased from 18.1% to 18.8%. That’s a marginal change and it’s in the positive direction. It pretty much balanced out the change in BB rate.

GB/FB ratio – decreased from 1.73 to 1.60, which represents an increase of just 1.1 percentage points in fly balls. That’s about .5 a fly ball per start. That’s not much but it’s something to note.

BABIP Allowed – increased from .279 (below average) to .302 (around average). Ok, that kind of change would lead to more runs. But 1.33 per game?

HR/FB – jumped from 9.2% of fly balls leaving the park to 16.1%. Woah! Bingo! I think we have our answer.

It’s pretty obvious what happened in 2019. Mikolas was the same pitcher as 2018. Except that he gave up a marginally higher percentage of fly balls. He had more men on base when those fly balls happened – BABIP increase. Then he experienced a huge increase in the percentage of fly balls that left the park. Add it up and you can perfectly explain the difference in runs allowed. That’s what happened. What does that mean for Mikolas’ future? Let’s talk about the history of HR/FB rate for Cardinals starters. In 2018, Mikolas was superb at keeping fly balls in the range of fielders – both at home in power suppressing Busch and also in the less-friendly confines of the road.

Mikolas’ 2018 HR/FB rate:

9.2% overall
Home – 7.4%
Road – 10.9%

Compare that to what happened the next season:

Mikolas’ 2019 HR/FB rate:

16.1% overall
Home – 11.2%
Road – 21.5%

Nearly two-thirds of all of Mikolas’ homers allowed in 2019 came on the road – 10 at home vs. 17 on the road (27 total). HR/FB rate is somewhat controllable by both hitters and pitchers, but there’s also some luck involved. What part of the park was the ball hit? What was the wind direction that day? Was Jose Canseco one of the outfielders? All of that factors in. No matter what a pitcher does, though, a certain percentage of balls hit into the air will carry over the fence. It’s just physics.

Mikolas’ 11.2 HR/FB% in Busch in 2019 is significantly higher than his 2018 rate, but it’s still pretty good. That rate feels to me like regression more than poor performance. I’m going to leave that one alone and call it real.

That road rate, which wildly influences his overall HR rate and ERA, feels off. Sure, Mikolas was hit harder – rise in BABIP – but it just seems fluky for almost all of that additional damage to come on fly balls that leave road ballparks. Busch suppresses power but it doesn’t do it at rates even remotely close to what that HR/FB road vs. home implies. Or does it?

That led me on a little statistical adventure to discover the typical range of HR/FB rate for Cardinals starters. My theory was that knowing the history of the HR/FB rates among Cards starters, playing half their games at Busch, should help us determine how much of Mikolas’ change in HR rate (road and home) is luck vs. likely. In other words, could Mikolas come back in 2021 and perform the same as he did in 2019 – which was essentially the same as what he did in 2018 – and see a significant decline in HR rate, and therefore, ERA and FIP?

So, I looked up the HR/FB numbers for Cardinals starters from 2000 to 2020. These are the relevant returns:

2020 - 15.9%
2019 – 16.4%
2018 – 10.3%
2017 – 14.9%
2016 – 13.0%

From 2010-2015, the Cardinals’ HR/FB rates were shockingly low. StL starters did not have a cumulative HR/FB rate above 10%. While those 2010-2015 numbers are intriguing for this argument, I think we have to throw them out for now. Through 2020, home runs were up league-wide. That shows in the pretty consistent rise in rate from 2016 on, with 2018 standing out as a nod to the past – and a cautionary note about projecting this trend without recognizing the potential for variance.

Speaking of variance, 2020 was a short season but it is also informative. The starter’s HR/FB rates were lower in 2020 than the year before even though the Cardinals were forced to use 10 different pitchers to start games and several – including Martinez (26%), Flaherty (23%), Ponce (19.5%), and Woodford (100%) – allowed extremely high HR/FB rates in short samples. Those four, forced to pitch under the most unusual of circumstances, represent 38% of the starts made by Cardinals starters last season.

So, HR rates dropped in 2020 when they probably should have been worse. Maybe that 16.4% rate isn’t a trend. Maybe it’s a data spike? An outlier that’s also influenced by extreme performances?

We’ve already described Mikolas’ road rate as unusual. Michael Wacha and Dakota Hudson were both over 20% that season. Those two account for the lion’s share of the increase witnessed in 2019. Both also seem like HR/FB outliers when compared to the rest of the Cardinals starters over the last 20 years.

Now, what about 2021?

The Cardinals currently sport a 9.7% HR/FB rate. That’s a huge drop, reminiscent of what happened in 2018. It is closer to what the Cardinals experienced for the first half of the decade.

Gut-instinct says that’s going to increase. A ton. And soon. Hold your horses… or lizards.

The 2021 drop in HR/FB rate is coming in a season where baseball executives decided BATTERS SUCK/PITCHERS RULE so they changed the ball. 20 teams have rotations that are allowing a 15% HR/FB rate or less. For context, just 11 teams did so last year. 13 did it in 2019. The Cardinals aren’t the only team that’s seeing fewer balls leave the park. It’s pretty universal.

Unless that changes, we should expect the Cardinals to post a significantly improved HR/FB rate. The club peaked in the 16 range, which might have been a statistical blip, but had decades of rates at 12% and below. It’s not that unreasonable to think the 2021 Cards will finish in the 10-12% range.

Take that logic back to Mikolas. I took Mikolas’ normal stats – a normal K and BB rate and left his 2019 BABIP alone. Then, I started calculating his ERA based on a range of possible home runs per flyball allowed rates.

HR/FB% = ERA
16% = 4.16
15% = 4.01
13% = 3.82
11% = 3.52
9% = 3.28

That should reveal just how much impact HR/FB rate had on Mikolas’ regression in 2019. With BBs and Ks relatively stable, the ~.020 normalization of his BABIP (i.e. more and harder contact) accounted for about a half run of ERA – 2.83 to 3.28 at the same HR/FB rate. That passes the math smell test. The rest of his regression is tied directly to his unusually high road home run rate.

Where will Mikolas land this season?

Steamer and ZiPS – computer projections – both reached the same conclusion I did. They believe in his rising strikeout rate (to some extent) and still think he’ll be elite in limiting walks. However, they both leave his HR rates alone and conclude he will pretty much stay what he was in 2019 this season.

There’s no way that a healthy Mikolas allows the same HR rate that he allowed in 2019 in this run environment. Preseason, I wanted to put a then-healthy Mikolas in the 13% HR/FB rate, which would give him an ERA in the 3.80 range, and an fWAR over 3.0 if we assumed 180+ IPs.

If Mikolas had not had a setback in his recovery, I would probably tweak that projection down based on the decrease in homers league-wide.

He did have a setback, though. His recovery might sap some of his command and control – we might see more walks this season and maybe even fewer strikeouts. But the playing environment will help him in the end, just as it has helped Gant, Kim, and Martinez. He’ll straighten things out by July and the final result will be in that same 3.80 ERA, with an fWAR adjusted down to 2.0 over 2/3’rds of a season of innings. (And probably less as the club protects him.)

Who wouldn’t take that? Welcome back, Mikolas! It’s going to be great to have you in the rotation again.

Enjoy your Saturday.