2021 Review — Starting Pitcher xK% Underperformers

Let’s return to our review of 2021 performances, this time flipping over to starting pitchers. Back at the beginning of 2017, I revealed the latest version of my pitcher xK% equation. From an adjusted R-squared perspective, it was the best equation I had developed. I’m never satisfied though and decided to perform my annual xMetric review over the winter, which included xK%. Lo and behold, I was able to develop an even better equation, which seemed impossible, but nonetheless, actually happened.

Rather than dedicate an entire post to the new equation, I will quickly discuss it and then get to the 2021 underperformers. From the original equation, strike percentage has been kicked out. In its place are two new variables — Pit/PA and 3-0%. Pit/PA is pitches per plate appearance and it strongly correlates with strikeout rate with a 0.55 mark. 3-0% is percentage of plate appearances that reach a 3-0 count. That variable correlates far more strongly with walk rate, of course, but has a small negative correlation with strikeout rate at -.09. The shuffling of variables changed the coefficients of the variables that have stuck around.

The new pitcher xK% equation is as follows (all metrics from baseball-reference.com):

xk% = -0.6635452 + (Pit/PA * -0.083200921) + (L/Str * 1.730740378) + (S/Str * 1.883371341) + (F/Str * 1.531285522) + (3-0% * -0.299250271)

Adjusted R-squared = 0.944

Now let’s dive into the pitchers who most underperformed their xK% last year. I filtered for pitchers that I have Pod Projected for at least 1 game started this season.

Reid Detmers is the only projected starting pitcher to have underperformed his xK% by more than 3%. Detmers was the 10th overall pick of the 2020 Amateur Draft and rapidly ascended through the minors. In fact, he recorded 54 innings at Double-A last year and then only pitched six innings at Triple-A before getting the call. Though it came over a rather small sample, he so dominated the minors, driven by insane strikeout rates, that expectations were quite high for his debut. Sadly, he completely flopped. I could forgive his 7.40 ERA as ERA is pretty meaningless over a small sample of 20.2 innings. But the 18.8% strikeout rate was a massive disappointment. His 22.2% xK% is quite a bit better, but I think we all expected better than that as well. He did generate swinging strikes at a better than league average rate, but not significantly so like he did at Double-A. The real problem was a lack of called strikes, and a lower than average foul strike rate.

Detmers’ best weapon was supposed to be his curveball, which was graded 65/70. He threw it 26.1% of the time with the Angels, but it only generated a 12.1% SwStk%. That would be okay if the pitch generated a high rate of called strikes too, but I’m not sure if called strike rate per pitch is available somewhere to check. Given his overall underwhelming called strike rate, I am assuming it wasn’t particularly strong. On the other hand, his slider and rarely used changeup both generated above average SwStk% marks. As an extreme fly ball pitcher, he’ll need those strikeouts to keep runners off base when the inevitable fly balls sail over the wall. Expect better in 2022, but with at least six starting pitchers ahead of him on the depth chart (according to our RosterResource page), it’s anyone’s guess how much fantasy value he’ll contribute this year.

Miles Mikolas missed the 2020 season due to undergoing Tommy John surgery and then significant time in 2021 due to tightness in his right forearm. However, his xK% have remained extremely stable during his first three seasons back in the Majors, so his 2021 actual strikeout rate sticks out as the largest underperformance. That makes sense given the small sample size of innings it came over. Still, he’s not much of a strikeout guy, but with excellent control, a ground ball tilt, and a friendly home park, he’s a boring, but acceptable fantasy contributor.

Yikes, it has not been a very encouraging first 74 innings for former top prospect Spencer Howard! But xK% gives us a bit of hope, though it isn’t being driven by an above average swinging strike rate. His better than average xK% is due largely to lots of called strikes, which are less repeatable than the swinging variety. One thing he may need to do is settle on a simplified repertoire. According to our Pitch Info Pitch Types, he threw six different pitches last year! Aside from the need to induce more whiffs, his control needs to rebound and walk rate drop back into single digits, especially given his fly ball tendency. He’s a crapshoot at the moment.

Since becoming a member of the Diamondbacks and seeing his strikeout rate collapse in 2020, Madison Bumgarner has now dramatically underperformed his xK% two seasons in a row. Before 2020, he had either matched or overperformed his xK%, so one wonders whether the last two years of underperformance is a fluke or he’s now doing something (or not doing something) missed by my equation. His fastball velocity was way down in 2020, but it partially rebounded in 2021, so I’m not sure that’s the excuse. Whatever the reason, the skills right now are completely meh and his fly ball rate has gotten extreme where he needs those strikeouts to return, or that’s a lot of multi-run dingers.

Quite the comeback year for Alex Wood! His strikeout rate jumped to a career high as his sinker velocity increased to its highest since 2017. Incredibly, xK% suggests his strikeout rate should have been higher. It wasn’t only more swinging strikes, but also the highest called strike on my file going back to 2015. It’s hard to believe that’s repeated, so I wouldn’t bet on another mid or high 20% strikeout rate. That’s fine though given his solid other skills and forgiving home park.

Elieser Hernandez was a sleeper for many heading into 2021, but he battled biceps inflammation and then a strained quad, limiting him to just 51.2 innings. He was decent though and xK% suggests he should have enjoyed even better strikeout results. That was mostly due to a higher than average foul strike rate, which is the least repeatable strike type rate of the three. He’ll need to bump his swinging strike rate up to get me more interested.

A.J. Alexy hasn’t posted a single digit walk rate since a tiny sample of innings in Rookie league back in 2016, so that’s all you really need to know about his fantasy prospects.

Oh Dylan Bundy, how you made me so sad last season. After a true breakout season (well, third of a season) in his first year on the Angels in 2020, Bundy was at his worst last year and was eventually banished to the bullpen. His xK% suggests things shouldn’t have been that bad, but a slightly higher strikeout rate certainly wouldn’t have changed his inflated HR/FB rate. He did reverse the trend of falling fastball velocity, but now it seems pretty clear it wasn’t home run friendly Oriole Park inflating his HR/FB rate, as he posted a career high mark with the Angels. I don’t know what to think anymore, but he’s someone who needs to show a velocity bump to get me interested again.

Sammy Long showed some strong strikeout rates in the minors, but his debut didn’t exactly go as hoped for. His xK% looked better thanks to a ton of foul strikes, which may very well not occur again. That swinging strike rate is looking quite lowly, so he’ll have to prove his stuff can translate to the Majors.