Leaderboard of the Week: September K%-BB%

This week's simple leaderboard looks at starting pitcher performance in September to find any possible breakouts. To find them, I took all starters with at least 20 innings pitched in the final month of the regular season and ranked them by K%-BB%. When looking at these small samples, the key is to identify any hidden talent change that can possibly carry over to next season. 

Here are the top 30 performers, with my thoughts on some of the most interesting guys:

Eury Perez: Perez returned to the majors in early June after having Tommy John surgery in 2024. His overall results were acceptable and in line with his previous major league season (4.24 xFIP vs 4.07 xFIP).

The 22-year-old started to improve in mid-August when he adjusted his pitch mix. Here are his pitch mixes and results both before August 18 and then since that day:

I'd like to see him increase his changeup usage, as the pitch posted a 17% percent swinging strike rate last season.  

It'll be interesting to see how he gets valued in early drafts. Hopefully, people are scared off by his 4.88 ERA in September and don't dig into his underlying skills. 

Simeon Woods Richardson: Really. Woods Richardson and Elder are the two I didn't expect to see in these rankings. Coming into September, Woods Richardson had a 4.59 ERA (5.22 xFIP), 1.42 WHIP and just a 9.3 percent K%-BB%. While he dropped his walk rate somewhat in September (from 10 percent to nine percent), his strikeout rate went from 23 percent to 35 percent. 

His big adjustment was upping his splitter usage from around 10 percent to nearly 30 percent. The splitter's results were great, with an 18 percent swinging strike rate and a 53 percent groundball rate. During that same stretch, he dropped the usage on his useless curve (five percent swinging strike rate, 46 percent groundball rate) from 15 percent to six percent. Finally, he tightened up his release point over the last month, leading to more consistency. 

Solid late-round sleeper. 

Kyle Bradish: Straight up shoved (2.53 ERA, 2.67 xFIP, 13.2 K/9, and 1.03 WHIP) after returning from the IL. There is an argument to be made that he's a top-20 starting pitcher if the results are even somewhat repeatable.  

Colin Rea: The 35-year-old put up some amazing numbers in September, but I couldn't find any major change driving his improvement. No velocity bump. Same pitch mix and usage. He even got hit harder, as his hard hit rate and barrel rate were both up. Sorry. 

Casey Mize: Mize was a cromulant, 4.00 ERA talent (with a 15 percent K%-BB%) for most of the season and then turned it on in September against a solid set of opponents (NYM, NYY, CLE, ATL, BOS). 

The only adjustment was nearly ditching his slurve (eight percent usage) and leaning into his slider and elite splitter (which had a 19 percent swinging strike rate). 

Mize needs one of his non-splitter secondaries (curve, slurve, slider) to become average or better. Almost no pitchers are elite with both a fastball and a splitter. That said, if he does add a good pitch, he could take off. Pay for the boring 2025 results and hope he's one small tweak away from a major step forward. 

Shane Smith:  While Smith had an amazing September, he actually made a major adjustment about a month earlier, changing his position on the pitching rubber by half a foot. Before the adjustment, he posted an 11.1 percent K-BB%. After the change, his 19.5 percent K-BB% puts him in company with Dylan Cease, Nick Lodolo and Nick Pivetta.  

Smith showed a solid arsenal, with four pitches with an 11 percent or higher swinging strike rate, including his four-seam fastball. 

Another solid sleeper. 

Bryce Elder: Elder struggled for most of the season, finishing with a 5.30 ERA (4.04 xFIP) and 1.39 WHIP. It was a rollercoaster of a season with two good months (May and September) and struggles in the other four. When he didn't walk batters, he thrived:

The rollercoaster ride hasn't been limited to just this past season. Here are his ERA and walk rate over the past four seasons:  

He's shown good and bad stretches. The problem is the extreme values, especially on the high end.

Yu Darvish: I'm surprised to see Darvish on this list since he struggled on the surface (5.38 ERA) after returning from the IL. He wasn't horrible otherwise, with a respectable 1.18 WHIP. All the ERA damage came from a 1.8 HR/9 and 66 percent strand rate.  

The 39-year-old righty struggled to get lefties out, managing just an eight percent K%-BB% against them compared to a 26 percent K-BB% against righties. Also, he had issues once runners were on base, posting a 20 percent K%-BB% with the bases empty and an 11 percent K%-BB% with a runner on.  

He slowly improved his strikeout (6.8 K/9 to 9.0 to 9.6) and walk (3.4 BB/9 to 2.3 to 1.5) numbers each month. 

One issue surrounding Davish is that he might not return next season.

Preller said he's talked with Yu Darvish "quite a bit over the last week," but the starting pitcher does not appear to have indicated anything about his future in the organization. Darvish, 39, is under contract through 2028, but he spent the first half of this season on the injured list, and said he would pitch without worrying if his elbow would "break" in the postseason and would need the offseason to think about what it would take to pitch next season. Said Preller: "We'll keep talking to him over the course of the next couple of weeks to see what it all means for him. … He's a big part of the organization, and I think we'll have more conversation here over the course of the next couple weeks, but it's just really been the start of it overall."

I don't expect Darvish to cost much with all the bad news and results surrounding him, but pitchers have rebounded from worse. 

Quinn Priester: I suspect Preister will get quite a bit of love thanks to his 13 Wins and 3.32 ERA from last season. He saw a nice boost in his strikeout rate, which went from 19 percent through the end of August to 25 percent in September, primarily thanks to the Angels. 

When I started looking into these pitchers, I expected to find more guys boosted up by the Angels, who had a team 31 percent strikeout rate in September. Try to guess the September game in which Priester faced the Angels: 

Removing the Angels start and Priester would have a 21 percent strikeout rate for the month, not enough to make the list.