Caleb Thielbar's Cubs Tenure Is Ending With A Whimper

After seven strong innings from the new, nominal ace of the Chicago Cubs, Kevin Gausman, Craig Counsell turned to Caleb Thielbar in the eighth inning of Friday night’s game to protect a 6-2 lead. Four hitters later, the score was 6-3, and the Kansas City Royals had runners on second and third with just one out. 

Thankfully, Trent Thornton then entered the game and put out the fire, allowing just one more run to score in a game that the Cubs would go on to win 6-4. No harm, no foul, right?

Friday was the third straight game that Thielbar worked out of the bullpen. Clearly, Counsell thought the veteran could handle it after throwing 19 pitches on Wednesday, and just 12 on Thursday. After all, this is a taxed bullpen that was working their 15th game in 15 days. 

The flip side of the coin, though, is that Thielbar gave up four runs to the Dodgers in his outing on Wednesday. Frankly, the left-hander has been outright bad since returning from an injury on May 19: he has a 6.00 ERA in 27 innings since then. Was he really the best option for the Cubs on Friday night?

The easiest place to start with Thielbar’s struggles is with his performance against left-handed hitters. As a left-handed relief pitcher, getting lefty hitters out is a must. 

Thielbar has made a career out of this. Until, basically, now. All stats courtesy of FanGraphs:

By almost any measure, Thielbar has actually performed worse against lefties this season than righties, which is bad when, again, retiring left-handed hitters is his primary job as a southpaw out of the bullpen. 

Thielbar attacks lefties with four pitches: his four-seam fastball, a slider, a sweeper, and a curveball. When a pitcher shows signs of decline, particularly a 39-year-old pitcher, my first assumption is that he is losing velocity. That’s not the case here: he is actually throwing his fastball and curveball a tad bit harder, while his slider and sweeper are down about a tick or so, according to Baseball Savant. 

Let’s take a look at his results on his three primary pitches, the four-seamer, sweeper, and slider, against lefty hitters. All stats courtesy of Baseball Savant:

Despite the slugging percentage against the sweeper, it’s the slider that I really want to hone in on. He’s upped his usage of it against lefties, throwing it 27.6 percent of the time compared to 17.5 percent last season. One look at his pitch locations, and it’s easy to see what the issue is:

That red zone just off the plate and away from the left-handed batter that occupies so much of the 2025 graphic is all but gone on the 2026 version. In 2025, 56.1 percent of the southpaw’s sliders to lefties were in that shadow or chase zone away from lefties. In 2026, that has plummeted to 27.9 percent.

That means a whole lot less of this:

And a whole lot more of this:

With baseball, and pitchers in particular, it is hardly ever just one thing. All four home runs Thielbar has allowed against lefties have come against the fastball and the sweeper. Still, it’s easier to sit on those pitches when the slider is either an easy take way off of the plate, or a meatball right down broadway. 

This doesn’t even get us into his struggles against righties, either. The more I think about it, the more I wonder if Thielbar being used three days in a row was a bit of a last gasp. A chance for him to show that he still belongs, and an opportunity for the Cubs to get as many innings out of him as they can while he is here. 

Hoby Milner and Daniel Palencia are both due back soon and will need roster spots. Tyler Ferguson has at least shown that he deserves some more chances. A roster crunch is coming soon, and if Caleb Thielbar can’t even get lefties out, he just might find himself on the outside looking in.

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