Casey Mize's Slider Has Become A Problem Padres Can't Ignore

Before this season, Casey Mize’s slider had never been his most reliable pitch. After allowing a .529 slugging percentage in 2024, he tried to get it back in 2025. The adjustments led to an interesting improvement in his Vertical Break from 0.2 to 4.5 inches. This season, Mize began giving his slider more presence in key situations. He increased its usage by 11.0 percentage points, and kept the opposition under control while his fastball and split-finger continued to function as his most dominant pitches.

However, the story began to take a turn after June and, after arriving in San Diego, the slider has become a problem for Mize and the Padres.

The most concerning points here lies in opposing hitters’ ability to connect with pitches outside the strike zone. They are not doing it with more swings. Since June, O-Swing% has fallen by 9.5%, but hitters have posted a 16.7% increase in O-Contact%. Mize has increased his slider usage by 7.0% since joining the Padres’ rotation in August, and opponents have raised their O-Contact% rate to 66.7%, the highest in any month this season.

Mize has also stopped generating swings and misses. His SwStr% rate peaked at 11.7% and 17.2% between March/April and May. Over the last two months, it has fallen from 10.3% in July to 5.4% this month. There is one key point that may be at the root of the situation: the Vertical Drop on Mize’s slider has declined from 36.8 inches in his dominant 2024 season to 30.3 this year.

Left-handed hitters, in particular, have taken advantage of Mize’s command lapses to produce better at-bats against his slider.

As expected, Mize has stopped trusting his slider, limiting its usage with two strikes this month to 17.9% compared to the pitch distribution in July. That answers part of the reasons why we have seen his K% fall by 17.4%.

That said, the danger has been present when left-handed hitters have put the ball in play over the last two months. To begin with, look at the 15.0% increase in Ideal Attack Angle. That says a lot about how well hitters are reading his pitches when attacking the slider. Then, the result of the expected metrics tells us the rest of the story: left-handed hitters have adjusted in such a way that they have raised their BABIP by .232 points, while cutting their swing-and-miss rate by a significant margin. There are almost no differences in HardHit%, but even without increasing the quality of contact, the production has been much higher across all expected metrics.

Here we have the xSLG averages on the left during the initial stretch between March/April and June against left-handed hitters. Then, the rest of the season.

Hitters are crushing the slider from the middle of the zone toward the lower outside corner. They have also gotten devastating contact in the lower inside part of the zone. Mize’s slider has only had a little life when it stays in the upper part of the zone or moves away. During the first months of the season, that was also where the most dominant trend was marked.

Mize will have to choose a new plan to sustain his repertoire. He has the weapons, but his slider is definitely not at the level it needs to be to prove that the Padres’ bet on strengthening their rotation at the trade deadline was the right one.