Caleb Thielbar's Curveball Was Perfect Monday Night, and He's Been Close to That Since Mid-April

[Matthew Trueblood]

Image courtesy of © Kamil Krzaczynski-Imagn Images

Some old dogs absolutely refuse to do anything but learn new tricks. They just keep learning new tricks, over and over, until even young dogs go, "Dude, chill." They're a rare breed, but when you're rounding out the pack of competitors in a modern playoff-hopeful bullpen, you badly need that kind of old dog. Caleb Thielbar is the alpha in that pack.

Thielbar, 38, made it to the majors by way of independent ball. A Minnesota native, he went to South Dakota State University (hardly a baseball powerhouse) and was taken by the Brewers in the 2009 MLB Draft. He pitched for them at their complex in Arizona and in Helena, Mont., with their Pioneer League affiliate. In 2010, he split his time between Helena and High-A Wisconsin. After that year, the Brewers cut him, and at 24, he had his first chance to give up on baseball.

Instead, he took up with the then-independent St. Paul Saints in 2011. The Twins spotted him while he was there, and signed him to a minor-league deal in August of that year. He steadily climbed through their system, and debuted in May 2013—more than three months past his 26th birthday. He was a solid reliever for his hometown team for the balance of that year and the bulk of 2014, but then things fizzled for him in 2015. He spent most of that season in the minors, and all of 2016 and 2017 back with the still-independent Saints. This was his second chance to give up on baseball.

Thielbar got back into the affiliated ladder in 2018, though, signing with the Tigers. He spent all of that year and a chunk of 2019 in Detroit's system, pitching in Erie, Penn. and Toledo, Ohio, and then a bit of 2019 with the Triple-A Gwinnett affiliate of Atlanta. He put up some rather dazzling numbers, especially in the latter campaign, when he struck out 94 batters (29.5% of those he faced). He was less smoke-and-mirrors than he had been when he first scratched and clawed his way to the majors, by then. Counterintuitively, he was finding more and more raw stuff as he worked at his craft. He'd first come up as the guy wholly reliant on location and guile, and was tapping into nastier offerings in his early 30s.

Still, he hadn't gotten back to the majors, and the time seemed ripe to hang it up. On the occasion of that third opportunity to give up on baseball, the 32-year-old was ready to do so. He had already taken a job as a pitching coach at a tiny Div. II school when he decided to take a last job as a doer of the job instead of a teacher, toiling for Team USA in a tournament in Arizona. He was so good there, though, that the Twins (and some other teams) wanted him again, after all. So, he came back to the game once more.

Thielbar then hooked up with Driveline (only then, at 33), which helped vault his stuff even further forward. That's how, for instance, this happens.

Because otherwise, to put it simply, this doesn't happen. Pitchers do not debut in the majors on the threshold of their late 20s, throwing 89-90 mph, and then steadily gain until they're sitting 93 and touching 95 at age 37. That's what Thielbar has done, though. It's how he carved out a slightly longer-lasting role in the Twins bullpen, and how he finally got to six years of big-league service time (yes, that's really all) at the end of last year, allowing him to hit free agency.

It's not just the velocity, though. The remarkable thing about Thielbar is how willing and able he has been to adapt himself to pitching in different ways and different roles for so many years, and that's continued apace throughout the second act of his career. It's continued, and arguably even accelerated, since he joined the Cubs over the winter.

Monday night was a bit of a crowning achievement. Yes, it was just an inning of work in a seven-run game, but Thielbar mowed down the Giants on 10 pitches. Half of those were curveballs, and his curveball was perfect. No, really, it was perfect. He threw five hooks. Three of them induced whiffs, and two went for called strikes. Since the start of the 2023 season, only three pitchers have used a curve at least five times in a start without any of them resulting in a foul, a ball, or a ball in play. Ronel Blanco and Nick Martinez each did it once last year, and then there's Thielbar.

It's not some galloping shock that Thielbar's curve is effective. It's the pitch that first got him to the majors, back when the fastball was nothing to write home about and there was nothing else worth mentioning in his arsenal, even if you were already writing home about something else. Thielbar's curve was a high-spin thing, back when spin rate was cutting-edge data, not even yet available to the public. It's waxed and waned over the years, though, and was less vital to his attack last year—partially because some of the spin on it dissipated with time.

Well, the spin is back this year. Thielbar has cranked it back up from 2,487 rpm on average in 2024 to 2,621 rpm this year. It's also a whopping 1.5 mph firmer, and is dropping an extra inch en route to the plate. Put all that together, and it's little wonder it had such a good night Monday. In fact, it's not even surprising that the pitch has yielded a 50% whiff rate on opponents' swings this year.

The tempting rise and hard bite of that pitch plays up, too, because Thielbar has also added an inch of carry to his fastball this year. Opponents can't as readily assume that if the ball appears to pop up out of his hand, it's going to be the curve; the fastball is doing more of that too. At an age when most pitchers lower their arm slots by several degrees, Thielbar has pushed his higher than ever—all the way to 55°. That makes the fastball-curve combination more deceptive, particularly given the spin profiles on each offering.

By themselves, though, those pitches wouldn't be playing as well as they are lately. The heater isn't hot enough, and the curve separates from it so much that there should be some degree of diminishing return on their divergence. The breaking ball is clearly plus, but the fastball isn't, at least on its own.

This is the part where we talk about Thielbar not quite adding a new pitch, but making a seldom-used one much more prominent in his arsenal this year. The slider you see depicted there is an offering he threw less than 1% of the time last year, and less than 6% of the time in 2023. He's had the sweeper (his best pitch, according to Baseball Prospectus's StuffPro) ever since returning to the majors in 2020, but the slider has been little-used and low-priority. That's history, now. 

Though classified as a slider everywhere I've checked, this newish offering is functionally a cutter, for Thielbar. Here's what his spin profile looked like in 2024, with measured spin direction on the left and observed movement direction on the right.

The sweeper is a nasty pitch, but works mostly against lefties. Meanwhile, to get righties to offer at the curve more often and lefties to swing at the sweeper, he needed a pitch that would make it harder to distinguish the heater from the breaking stuff. Here's the same pair of charts for 2025.

A pitch thrown with that kind of true backspin and just a bit of movement created by non-spin direction effects is almost always labeled a cutter. I can only surmise that Thielbar's tricks the system because of the depth he achieves on it, relative to his fastball, and because of his extreme arm angle, but the pitch comes in at almost 88 mph, too, so there's not much of a case to be made for calling it a slider. It's his cutter, and it helps him keep hitters on their back foot against the curve.

Thielbar is also throwing the curve more than he has at any point in his career, give or take—and the fastball less than ever, unequivocally. 

With a true four-pitch mix for the first time in his life (at age 38), Thielbar can get very nasty with opposing hitters. Unlike most 38-year-olds, he's not just gutting it out on the ghost of his former stuff. He's throwing as well as ever, and it's harder than ever to figure out and hit his curve—which is bad luck for batters, since it's coming at them more than ever.

None of this should mislead you to think that Thielbar can be the Cubs' October relief ace, or anything. He's not that kind of acquisition. He'll either burn out before the postseason, or need a reset on the injured list at some point. Even the ageless lefty has to get old eventually, and for Thielbar, that's shown up over the last few years largely in the form of nagging injuries. Sometimes those have landed him on the shelf. At other times, they've merely led to inconsistent performances.

Either way, there will be more adversity ahead for Thielbar. He got off to a rough start this year, but he's been thoroughly locked in over the last three or four weeks. With his widened arsenal and tweaks in spin and movement, he can still get outs—even high-leverage outs—for a World Series hopeful. Spamming secondary pitches often works even better in October than throughout the regular season, and Thielbar seems well-suited to that gambit. Monday night was proof of concept: Thielbar can be a devastating weapon in an improving relief corps for the Cubs.

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[Bertz]

It's been really good to see him turn it around.  Early in the year he just could not miss a bat in the zone to save his life.  The zone contact rate against him through the second Dbacks series was a Madrigal-ian 95%, and very much *not* paired with a Madrigal-ian lack of pop.

Starting with the Phillies series he's been missing bats like crazy, 61% in zone contact.  He doesn't need to maintain that. or even come close.  But if he can split the difference and sit approximately little south of 80% i think all the other pieces are there for him to resume the quality work he's done the last few years.