A Blip? Or the Beginning of the End for Caleb Thielbar?

From 2020, when Caleb Thielbar made his triumphant return to the Major Leagues after four years in baseball exile, through the end of last season, only 22 relievers had a better strikeout-to-walk rate (K%-BB%) than Thielbar’s 22.7%. Only 19 accumulated more than Thielbar’s 4.08 win probability added (WPA) and 28 pitchers generated greater than Thielbar’s 3.1 fWAR.

Thielbar and his pitching lab-developed arsenal was one of the top 10% (or so) among the 223 qualified relievers over that time.

Unfortunately, Thielbar’s start to 2024 is making many of us wonder if the most important word in that sentence is “was.”

2024 Struggles

This season, over 24 appearances covering 16.1 innings, Thielbar’s K%-BB% has been chopped in half (11.9%), his WPA is -0.51, and his fWAR is -0.1. His ERA, which isn’t a great measure to use with relievers but remains our ubiquitous shorthand for pitcher effectiveness, sits at an unsightly 7.71.

There is a reasonable case to be made that Thielbar has just been enduring a rough patch, with some bad luck over a small sample, something that every player gets to experience at one time or another.

His batting average allowed on balls in play (BABIP) and runner strand rate (LOB%), two of our most commonly referenced numbers for assessing luck, support that cause.

Thielbar has suffered from a stunningly poor .415 BABIP which is 126 points above his career mark of .289. He’s stranded only 53.7% of the runners that have reached base against him, which is almost 21 points lower than his career’s 74.5% strand rate. Pitcherlist’s hit luck metric suggests Thielbar has allowed 15 more hits than we would expect from the quality of his pitches.

At the same time, some measure of Thielbar’s batted ball “misfortune” has been deserved. Opposing hitters are hitting the ball 90.5 mph on average against him this season, which is a little more than three miles an hour harder than his career average allowed. They are also doing that more frequently (hard hit rate 38.9% vs. 32.7% career) and on more advantageous launch angles (40.7% sweet spot vs. 35.2% career).

Also on the other hand, we’re approaching the point where he’s not dealing with a small sample size anymore. Yes, it’s only been 16 or so innings, but he’s pitched enough to have some belief in what the data is showing.

For instance, batted ball metrics like exit velocity become reliable around 45 balls in play. Thielbar has allowed 54.

Strikeout rate for pitchers becomes reliable somewhere between 75 and 125 batters faced, depending on the method used. Thielbar has faced 84 batters this season and has a strikeout rate of 22.2%, 8 points lower than the 30.2% rate he ran from 2020-2023.

Walk rate takes longer to stabilize, but Thielbar has already unintentionally given up first base for free more times this season (8) than he did in 36 appearances last season (6).

Those numbers should not be taken to mean Thielbar has forever lost his well-above-average ability to punch hitters out, command the strike zone, and suppress hard contact, but they do offer pretty good indications that something has changed for the worse.

Digging In

Let’s start the investigation with some high-level control and plate discipline metrics:

Data from Baseball Savant

Given what I shared in the section above, it should be no surprise to see Thielbar is throwing fewer pitches in the strike zone and batters are swinging and missing less frequently than they have in the past. Those both track with decreased strikeouts and increased walks.

That could be from a decrease in stuff, poorer command, or (usually) some combination of the two.

Thielbar has been operating with generally the same pitch mix and approach since his metamorphosis completed in 2020. He’s scrapped the gyro slider he toyed with last season and is back to working with elevated four-seamers about half the time and sweeping sliders and slow curveballs for the remainder. He’s also worked in favorable counts more or less the same as he has previously.

It’s (Mostly) About the Express

While all three pitches have performed worse this season, the missing whiffs and pitches in the strike zone appear to be mostly isolated to Thielbar’s fastball. Both his sweeper and curveball have whiff rates in line with or better than the past few seasons, and he’s throwing them in the zone at similar rates as in the past.

But the zone rate on his four-seamer has dropped from 58%-62% the prior four seasons to 53.7% this year, and the pitch has generated an empty swing just 13.8% of the time batters have gone after it, far below the 21.3% and 29.6% it created the past two seasons.

Given that from a 37-year-old pitcher now fifteen years removed from the start of his professional career, you might guess that Thielbar’s heater has lost a tick. But that is not the case. He’s averaging a career-high 93.2 mph this season.

While Thielbar has reversed the clock and gained velocity as he’s gotten older, the same kinds of improvements are not present in the other characteristics of his four-seamer.

The spin rate on the pitch, which contributes to movement (especially for fastballs), has declined year over year since the sticky stuff ban was enforced in 2021. Thielbar has averaged 2240 RPM this season, which reflects a loss of about 70 revolutions per minute since 2021. That might not seem like a lot on the surface, but it’s roughly a standard deviation lower than it was for him just a few seasons ago.

Velocity and spin rate are positively correlated (i.e., faster pitches tend to spin more). That Thielbar’s spin has decreased while his velocity has risen is another sign that the pitch isn’t quite the same.

That’s evidenced more directly when we look at its vertical movement. Higher spin four-seam fastballs fight the effects of gravity and drop less (or appear to “rise”) on their way to home plate. With less spin, the average induced vertical break (IVB) of Thielbar’s fastball has declined from 19.3 and 19.4 inches in 2021 and 2022, to 18.0 inches this season.

We might not perceive an inch-plus of lost break as an important deal, but when we consider that it’s about half the width of a major league baseball, we can see why it matters. That distance can be all the difference between a pitch getting whiffed and being fouled off or put in play.

To be clear, 18 inches of IVB is still an above-average figure, especially for a left-hander. It’s not like that’s now a bad amount of movement. But it’s not what it was. To that point, the public-facing stuff models, like Stuff+ and pitchingBot, have Thielbar’s four-seamer graded meaningful lower than they did previously.

It’s Also About Execution

Still, Thielbar’s fastball vertical movement was also diminished last season (18.6 inches) and the pitch still held opponents to .277 wOBA and was worth +3 runs. It seems unlikely the further decline in movement alone explains how it’s allowed .407 wOBA and accumulated -5 pitcher run value so far this season.

It’s also in some part due to command that hasn’t been as reliable as in the past. That’s obvious from the strike zone data I mentioned above, and we can visualize that with a comparison to last season:

From Baseball Savant

Where Thielbar’s fastballs were consistently landing up in the strike zone last season, you can see this year they are more sprayed around the zone. The overall heat map is wider and less concentrated. There’s a hotspot down. There are several misfires up and to his arm side. The elevated hotspot extends up above the zone.

The average height of Thielbar’s four-seamer has risen to 3.05 feet.

From Baseball Savant

He’s purposefully raised his sights over time because up is where his four-seamer will play best, but it’s fair to wonder if it’s now just too high, making it easier on hitters and less optimally paired with his secondary pitches.

I don’t think that’s been a purposeful change, but instead a difficulty in executing. Perhaps that’s a mechanical issue that can be ironed out. Maybe it’s mental. Or perhaps it’s some of both, which Thielbar’s recent comments to reporters after another tough outing would suggest:

“My stuff sucks. The process sucks. Everything just sucks,” said a very raw Thielbar. “I really don’t know what to say. I’m trying.

Embracing and Adjusting

Aging in baseball often follows somewhat predictable patterns. As hitters lose bat speed they often start to cheat on pitches and locations and start their swings a little earlier to catch up to fastballs which, in turn, causes them to chase out of the zone more often.

Pitchers compensate for diminishing stuff in a couple of ways. They can sell out for velocity and maximum nastiness (and sacrifice command) or they can embrace their new selves and double down on painting corners and out-executing their opponents.

That Thielbar’s resurgence was predicated so heavily on amped-up velocity and stuff (at least compared to her earlier career self), and his comments above, it’s fair to wonder if he’s sub-consciously started down the former path.

To that, it’s worth a reminder that he’s still working with a major-league-average arsenal (Stuff+ = 101). It’s down from what it was, but it’s not like it doesn’t belong and he’s “cooked” as Twins Twitter (TM) would tell you. But, he’s lost some margin for error and needs to make some adjustments to execute at a higher level.

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John writes for Twinkie Town, Twins Daily, and Pitcher List with an emphasis on analysis. He is a lifelong Twins fan and former college pitcher. Follow him on Twitter @JohnFoley_21.