Tyler Duffey and the case of the missing strikeouts

No matter how you look at it, the numbers say Tyler Duffey was one of the best relievers in all of baseball in 2019 and 2020. Over a combined 81.2 innings he pitched to a 2.31 ERA (2.91 FIP) with 0.94 WHIP, batters hit just .186 against him and struck out in 34.2% of their plate appearances.

His cumulative 1.9 fWAR ranked behind only Taylor Rogers among Minnesota relievers and ranked 13th-most among 161 qualified relief pitchers across the league. Duffey’s +2.58 cumulative win probability added (WPA) was best among Twins and checked in 18th-most league wide.

The 2021 season has been a different story.

Now through most of August, Duffey has a 3.55 ERA (4.10 FIP) with 1.45 WHIP and opponents have hit .238 against him. Those are not necessarily bad numbers — the league averages for relievers are 4.19 ERA, 4.22 FIP, 1.32 WHIP, and .234 batting average against — but they are quite different from Duffey’s most recent history. His 0.2 fWAR so far this season is tied for 93rd and his +1.20 WPA ranks 39th among 159 qualified relievers.

However you look at his 2021 numbers, Duffey has not been nearly as good as the past two seasons. This year he has profiled more like a generic reliever better suited for the early-middle innings, not the high leverage, late and close fireman we had seen previously.

With the Twins in evaluation mode for next season, I got curious about what’s behind Duffey’s change in performance. He’s 30 years old, making $2.2-million this season, and eligible for arbitration one more time this winter. Given that and his past and present performance, it is highly likely he’ll be part of the Twins’ bullpen again next year. But what kind of reliever can we expect him to be? Is his decline in performance a sign of things to come or something that can be turned around?

The first place I usually look when investigating a change in pitching performance is the fielding independent pitching inputs — strikeouts, walks, hit by pitches, and home runs. Ostensibly, these are mostly under Duffey’s control and any differences found here might help us pinpoint what’s going on.

Data sourced from Fangraphs.com

Right away, it’s easy to see that Duffey’s overall results are being hurt because his strikeout and free base rates have both moved in the wrong direction. The 13.8% walk plus hit by pitch rate is by far the worst of his career and the 20% strikeout rate is down near the level it was before his breakout.

On the positive side, he’s kept the ball in the park at a career best rate this season. Digging a bit deeper on his homers allowed I found that his 8.8% home run per fly ball rate is about half his career average. That suggests there is not a meaningful skill change in that area but rather he’s suppressing homers like before and benefiting from some good fortune this year.

Let’s dig deeper on the strikeouts and walks. Below is a breakout of Duffey’s rates of various types of strikes and balls:

Data sourced from baseballsavant.mlb.com

Generally, Duffey is throwing the ball in the strike zone at a similar rate as before. His called strike and foul ball rates are similar, too. But his swings and misses are way down and seemingly have been replaced with more called balls. This combination might suggest batters are not chasing after Duffey’s pitches out of the strike zone as much as before. Perhaps, instead of swinging and misses at those pitches out of the zone they are taking them for balls?

Sure enough, a quick check of his Statcast page confirms his chase rate this season is down to 22.7% after having been 32% and 34.7% previously.

That made me wonder if there is something different about Duffey’s stuff this season that is making it less tempting to go after?

To investigate that question, I started by looking at his pitch type mix. If he is deploying his pitches differently, that could result in fewer chases. But, I came up empty.

Duffey’s pitch mix in 2021 is nominally the same as the prior two seasons — 51% curveballs and 46% four seamers, with a tiny handful of two-seamers sprinkled in. There’s no real change relative to 2019 and 2020 there.

How about his velocity and spin stats? Changes there, especially declines, could certainly lead to fewer whiffs and chases. Let’s take a look:

Data sourced from baseballsavant.mlb.com

Duffey’s 2019 breakout was helped by a career best fastball velocity on his four-seam fastball. That declined last season, but did not seem to impact the pitch’s effectiveness — the .213 wOBA against his fastball in 2020 was supported by a .234 expected wOBA against as estimated by Statcast. That suggests him finding success with a slightly diminished fastball was not just a short season small sample fluke.

This year the velocity is mostly the same as last, but the results have been quite a bit worse, as you can see highlighted in red. The spin rate on his fastball is generally the same as it has been over the past three years.

The data above shows that Duffey’s excellent curveball seems to be the same as it ever was in terms of velocity, spin, and performance.

Maybe there is something different with how his pitches have moved?

Data sourced from baseballsavant.mlb.com

Here again, it does not look like much is different. Note that the movement measurements from Statcast are in inches and are inclusive of the effects of gravity. Duffey’s fastball maybe has lost some horizontal movement this year, but it’s still pretty similar to how it was in 2019. You might squint and say the curve has less vertical movement than 2019, but its worth remembering that Duffey worked with two different kinds of curves in 2019 and the slower version had more downward drop. If we account for that, his curveball is likely not moving much differently today.

I also checked on his release point and extension data and did not find significant change. At a high level, Duffey’s stuff seems to be the same as it was.

If he is largely working with the same stuff, yet getting fewer strikeouts, yielding more walks, and having his fastball get hit harder, we’ll have to get more granular.

Maybe Duffey’s challenge getting swings and misses on chase pitches is related to managing the ball-strike count?

Batters are more likely to chase pitches out of the zone when they are behind in the count. When the batter is ahead, they can be more selective in search of pitches to drive (usually fastballs). Perhaps Duffey is having trouble getting hitters into favorable counts?

Data sourced from baseballsavant.mlb.com

Here we might be on to something. Compared to the prior two seasons, Duffey has not been working ahead of hitters as frequently. His first pitch strikes are down almost ten percentage points. That has carried through to the rest of the counts and his overall percentage of pitches thrown while ahead in the count is down four percentage points from last season.

In general, batters perform far worse when the pitcher is ahead in the count. This season, the league is hitting .192/.202/.303 (.216 wOBA) when the pitcher is ahead and .282/.471/.507 (.426 wOBA) when the pitcher is behind. That is a massive difference.

Duffey’s numbers in these situations are more extreme than the league averages. When he has been ahead of hitters this season they have produced just .150/.164/.233 (.171 wOBA). When he’s fallen behind they have teed off to the tune of .313/.548/.479 (.459 wOBA).

Without the benefit of count leverage to put the hitters on the defensive, Duffey’s results on pitches out of the zone, which were previously among the best in the league, have tanked. Combining 2019 and 2020, Duffey threw 33.4% of his pitches in the chase and waste attack regions and allowed .217 wOBA and +3.8 total run value on pitches in those areas. This season, he’s thrown 34.1% of his pitches in those areas but allowed .443 wOBA and +14.6 runs.

The difference is largely attributable to batters taking these pitches. Duffey’s swing/take profile from Statcast reveals he has gotten swings on just 14% of the pitches he has thrown in the chase region and 3% in the waste region this season. The 2021 league averages in those regions are 22% and 5%, respectively. Last season Duffey’s comparable numbers were higher still — 32% and 10%, respectively.

For Duffey, working ahead is critical, because it allows him to expand the zone with his four-seamer up and his curveball down and force hitters to consider swinging at those pitches. When he’s behind, they can just ignore those offerings and wait for him to come over the plate. That he’s fallen behind more helps to explain how’s gotten worse results with stuff that is mostly the same.

As for what it means for the future, we can be encouraged that the data suggests he has not lost much quality on his raw stuff. His velocity, spin, and movement profiles are similar to the past. This suggests he could turn it around. But, to get back to being the dominant high leverage reliever he was in the past, he’ll need to find a solution for his inconsistent command and get back to working ahead of hitters much more frequently.

---

John is a staff writer for Twinkie Town with an emphasis on analysis. He is a lifelong Twins fan and former college pitcher. You can follow him on Twitter @JohnFoley_21.