Interesting Last 30 Day Starting Pitcher SwStk% Leaders — Through Jul 16, 2023

Pitcher skills change far more frequently than hitters and pitchers can seemingly transform themselves mid-season simply by changing up their pitch mix or adding or losing velocity. So it pays to not only look at a pitcher’s full season line when evaluating his performance and forecasting the rest of the season, but also how he has performed more recently. So with that in mind, let’s discuss some of the most interesting names on the last 30 day SwStk% leaderboard.

Eury Pérez | 17.5% SwStk%

Pérez was the Marlins top ranked prospect heading into the season, and third overall. With a 60/70 Fastball, 50/60 Slider, 45/60 Changeup, and 50/70 Command, his scouting grades suggest a potential dominant ace in the future. So far, one might argue that he has already been an ace over his 11 starts and 53.1 innings. He has posted a tiny 2.36 ERA and 28.4% strikeout rate, supported by an excellent 14.3% SwStk%.

He’s taken it up a notch over the last 30 days, which comprises four starts. In the first of those four, he generated an unbelievable 21.3% SwStk%! His last start featured a 15.1% mark and that was his lowest of the four starts.

Has he been doing anything differently? He sure has! He has dramatically upped his slider usage, at the expense of all his other pitches. The pitch has been darn good, generating an elite 21.4% SwStk%, but the crazy thing is that it hasn’t even been his best whiff pitch! His curveball has actually been even better, generating a 23.1% SwStk%. So the switch from curve to slider likely hasn’t been the driver of his increased SwStk%, but fewer fastballs and changeups likely explains most of it. Of course, a change in pitch mix might only partly explain the jump, as his pitches may have simply gotten even more whifftastic, but I don’t have splits on SwStk% by pitch and date to check.

I hate to put so much pressure on the 20-year-old rookie, but Pérez’s pitch mix and effectiveness looks eerily similar to Jacob deGrom.

Kenta Maeda | 16%

Maeda missed the entire 2022 season after undergoing Tommy John surgery and then returned this year to some very poor results over four starts in April. He then hit the IL with a triceps strain that cost him about two months.

He returned in mid/late June and looks like a completely new pitcher. He is throwing his slider more and his velocity is mostly back to normal, after sitting below 90 MPH over his last three starts before hitting the IL. And what a difference it has made to his SwStk%! Check out this chart:

While he was excellent in his first start when he still had his velocity, it all went downhill in games two to four, when he failed to average 90 MPH with his fastball. Since his return in June, his SwStk% hasn’t fallen below 13.3% and has been above 16% over his last three starts.

His underlying skills are now right back in line with where he’s been historically. That suggests that Maeda is healthy now and we should value him as such, rather than as a question mark returning from TJ surgery. With a 3.94 SIERA sitting significantly below his 5.50 ERA, he makes for an ideal trade target, as it’s very possible his owner doesn’t realize that Maeda is officially back.

Reese Olson | 15.5%

Olson was ranked as the Tigers 12th best prospect with solid offerings across the board, to go along with a standout changeup. The big red flag is his 30/30 Command grade, but control and command are seemingly easier to improve upon than the quality of a pitcher’s stuff. So that grade doesn’t scare me away. And amusingly, he sports an excellent 5.2% walk rate and better than average .255 BABIP, both metrics I would think would be hampered by a lack of command. Clearly, it hasn’t affected him yet.

Overall, Olson has been quite impressive, posting a 25.8% strikeout rate, backed by a 29.4% CSW%. His slider has been elite, generating a 21.2% SwStk%, while his 70/70 changeup has lived up to its hype with a 19.8% SwStk%. Oddly, he has thrown the pitch just 12.5% of the time.

His last 30 day 15.5% SwStk% is well above his 12.2% mark for the season. That’s because his last 30 days excludes his first two starts and relief appearance. Also worth noting is I filtered for just starters, which means his first two July appearances are excluded since he didn’t start the game. Those were his second and third lowest SwStk% marks of the season, so removing them from the average is boosting his last 30 day mark.

What’s surprising is that he has thrown his changeup even less frequently over the last 30 days and his four-seamer more often. That’s not a change you would expect to yield a higher SwStk%, especially when your changeup is supposed to be elite! He’s coming off the best SwStk% game of his short career at 18.7%, and it’s probably not a coincidence that he threw his slider the most frequently of any start.

With a mid-90s fastball, and two standout pitches in his slider and changeup, I like him now more than I expected to. We’ll just have to see if command issues crop up.

Tyler Anderson | 15.3%

Mr. Anderson was one of the most surprising names, as he ranked 12th among 147 starters in SwStk% over the last 30 days. Amazingly, Anderson has posted a double digit SwStk% every single season of his career. And he has been super consistent too, posting marks within a tight range of between 10.1% and 12%. So any run of a 15.3% mark is noteworthy given that he’s always been good, but not great, at generating whiffs.

Over his last 30 days, he has upped the usage of his fastball and reduced his cutter usage, and that’s really about it when it comes to pitch mix changes. Normally, you would expect a lower SwStk% when swapping the cutter for a four-seamer. One likely driver is a velocity bump. He has gained about 0.6 MPH on his four-seamer, nearly a mile per hour on his cutter, and 0.7 MPH on his changeup. So velocity increases across the board probably helped.

That velocity bump merely brought him back to where he sat last year, with the exception of his changeup, which he is throwing harder now. Personally, I don’t think these last 30 games is a better predictor of his future than his full season or his rest of season projections.

He’s 33, hasn’t changed his pitch mix much, hasn’t gained velocity versus last season, and is right in line with years past. I’m surprised he hasn’t performed better since departing Coors Field, but it’s clear that he is what he is. I wouldn’t chase this SwStk% streak thinking more strikeouts are coming.