Duffey put together a second half for the ages, giving up just a .208 wOBA, a .245 SLG and striking out 47 hitters in 29.1 second half innings. What led to Duffey’s surge to dominance? Here are three important steps he took in 2019.
Changing pitch mix and increased velocity
Duffey’s pitch mix has changed markedly from the beginning of 2018, as evidenced below (visual via Baseball Savant):
At the beginning of 2019, Duffey all but did away with his ineffective sinker, beginning to rely more heavily on his slider. In addition to ditching a pitch altogether, Duffey’s location began to shift dramatically. In 2018, Duffey was working primarily middle and down in the strike zone. This did not work well. In 2018, Duffey’s fastball was annihilated to the tune of a 1.011 OPS and 180 wRC+ against, making it a truly dysfunctional pitch.
In 2019, Duffey began to shift his fastball location up in the zone (visuals via Brooks Baseball).
Duffey’s fastball velocity is another data point of note. His four-seamer maxed out at 97.4 mph in 2019, almost three mph more than in his big league debut in 2015. This combination of increased velocity, elevation, and solid fastball spin rate (63rd percentile in MLB) transformed Duffey’s fastball into a strength. In 2019, opposing hitters managed just a .599 OPS and 66 wRC+ against it. In short, it became a dominant pitch.
The Development of a Super Slider
Duffey’s slider is unusual in that it has more vertical drop on average than any other slider in MLB (about 33% more than typical). Duffey’s slider performs opposite to that of Sergio Romo (who achieves a large amount of horizontal movement resulting in consistent soft contact). Duffey’s simply bottoms out, resulting in a 16.2 SwStr% in 2019. Indeed, Duffey’s vertical movement on his slider increased significantly in 2019 (visual via Baseball Savant)
Duffey’s increase in slider velocity (+2 mph in 2019) and his ability to command it had devastating effects. He gave up just a .257 wOBA and 69 wRC+ on the pitch in 2019 and took 150 points of opposing OPS off the pitch. Duffey now has two plus pitches, all you need to be an outstanding major league reliever.
Finally, Duffey showed outstanding command of his slider (image courtesy of FanGraphs). There is a tight clustering of the pitch down and away from right-handed hitters. Duffey rarely hung the pitch. When he missed with it, he tended to miss low and away. His ability to locate his slider down and away to right-handed hitters combined with it’s precipitous drop made it the second devastating pitch from a pitcher who began the season at Triple-A Rochester. After his impressive second half, Duffey finished in the 94th percentile in MLB in K%, and 85th percentile in xSLG.
Looking Back, Looking Ahead
The 28-year-old Duffey should remind Twins fans of a few truths heading into 2020. Player development is not linear. Much like Mitch Garver, Duffey’s rise to prominence should make fans throw out old nonsense notions about ‘age’, ‘development’ and ‘ceiling’. Secondly, he should reaffirm Twins fans faith in their front office, analytics and player development departments. Duffey has transformed into a modern reliever. A high velocity fastball with a solid spin located up in the zone and a wipe- out slider which he throws a ton. I’ll be interested to see if the Twins choose to add externally to their bullpen this offseason. I’ll be more interested to try and pick apart who the next internal guy for a makeover might be.