On May 18, 2017, Danny Duffy faced the Yankees at Kauffman Stadium, going seven innings while allowing no runs, striking out 10 and walking two.
On May 19, 2018, Danny Duffy faced the Yankees at Kauffman Stadium, going four innings while allowing five runs, striking out four and walking three.
Perhaps this is a good place to start when trying to figure out what's going wrong with the Royals' left-hander. Just 366 days ago, we have film of him dominating while throwing a scoreless seven innings; then, last week, we saw him continue his season-long trend of struggling to limit hard contact.
So what can we learn from comparing these two games?
One thing in particular seems to stand out most: Duffy's slider just isn't what it used to be.
That can be seen first in the numbers. Here is Duffy's batting average against and slugging percentage against for his slider in each of the last five seasons, via Brooks Baseball.
|Opp. BA||Opp. SLG%|
Opponents' batting average has risen nearly 100 points against the pitch from last year, while slugging percentage has more than doubled.
Based on pitch-value approximations, Duffy's slider has been worth negative-10.6 runs this season; to compare, the second-worst slider from an MLB starter this season has been worth negative-5 runs.
So what's going on here?
Perhaps it's best to start with what the pitch used to be. Duffy talked about his slider specifically in this 2016 Star story.
"(Hitters) have to respect the fact that it’s coming out of the same grip, the same slot, the same plane as my fastball,” Duffy said then. “But it’s got a last-second drop to it."
The final sentence is the key. Here's some 2017 video showing what he's talking about.
Notice where Chase Headley misses. He's expecting the pitch to stay up (like a fastball), but instead, the slider has a sharp break both down and away from his bat.
Eight of Duffy's 10 strikeouts in that 2017 game came with his slider, meaning the Yankees never were able to train themselves to swing to the correct spot on the diving pitch.
In 2018, that slider seems to have lost its tail.
Here's a comparison of the same pitch nearly a year apart. One bites back into the dirt, while the other runs directly into the bat of a right-handed hitter for a deep home run to left.
Pitchf/x data indicates minor changes as well. Duffy's slider has average horizontal movement .93 inches closer to right-handed hitters and average vertical movement 1.13 inches higher than last season. Movement in and of itself isn't inherently good or bad with pitches, but in Duffy's case, it might reflect that his slider's final nosedive isn't happening as frequently this season.
It's tough, without pitching expertise, to diagnose the exact issue. Going through the video frames of his compared pitches, there were small instances where his body went off track just a bit from last season. Here's one example, as his pitching hand seemed to be slightly higher a year ago.
It's difficult to tell on video, but his release point might also just be an inch or two lower than last year — something that Pitchf/x appears to be tracking as well.
There's so much that happens with each pitch, though. Here's a slow-motion look of Duffy's slider mechanics from the 2017 game against the Yankees.
Something had to be causing that late break in the slider, and notice how Duffy gives a late flip of his wrist that could contribute to that. Could that motion be tougher for him this year? Remember, he had an elbow injury late last year, and even if Duffy is as healthy as he suggests this season, there still could be something physically or mentally that is keeping him from the late snap that made his sliders dip a year ago.
This hasn't been Duffy getting unlucky. Based on Statcast data, Duffy is 186th out of 190 pitchers in the all-encompassing expected weighted on-base average, which takes into account quality of contact along with walks and strikeouts; last year, he ranked 205th out of 505.
The biggest change can be traced back to the ineffectiveness of one pitch.
Duffy, in one year, has seen his best weapon turn into his worst offering.
And without a fix, it seems unlikely that he'll have the tools needed to become the elite starter he's been before.