KC Royals: Is Danny Duffy Suffering From Fatigue?

KC Royals starter Danny Duffy laid claim to the “ace” role with a fantastic three month run from June to August. However, beginning with an awful seven-run outing against the Red Sox on August 28, he collapsed with a 6.49 ERA in his next six starts.

The drastic decline in Danny Duffy‘s performance, combined with exceeding his previous high of 149.2 innings pitched, suggest he might be fighting fatigue.

Duffy has pitched 173.1 innings with one more scheduled start in the season on Thursday (today). His average fastball velocity has dropped to a season-low 94.59 mph in September, down from 94.86 mph in his outstanding August and 95.13 in July.

A pitcher losing a little zip on his fastball in September is nothing unusual. In fact, most pitchers lose some velocity at the end of the season. However, Duffy’s walk rate also jumped to 2.7 BB/9, which is a significant jump from his 2.1 BB/9 season rate. That increase caused his strikeout/walk ratio to drop to a season-low 4.00 for the month, despite a leap in his K/9 rate to 10.2 (up from 8.2 K/9 in August).

The big thing that jumps out at me from Danny Duffy‘s Brooks Baseball data is that his fastball and sinker have become much more hittable in September.  His batting average against zoomed to .327 for his fastball and .500 for his sinker in September. That’s up from .196 for Duffy’s heater in August and .214 for the sinker.

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Duffy wisely reduced his sinker usage by half in September, but he’s actually increased his fourseam fastball usage in September to 39.17% (up from 36.92% in August). On the other hand, Duffy’s whiff rate and batting average against improved with his slider. Unsurprisingly, Duffy’s slider usage jumped in September.

Overall, he’s getting more strikeouts in September, but hitters are teeing up his fastball and sinker much better. He’s given up 8 home runs in 29.2 innings pitched in September, which is his highest monthly home run rate of the season. His slugging percentage against zoomed to .796 for his fastball and .900 on his sinker.

The puzzling thing is his movement data for his fastball and sinker don’t look much different between August and September. What I’d really like to see is the monthly breakdown on his spin rates. A declining spin rate on his fastball might explain his reduced effectiveness.

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However, the bottom line is that it’s hard to succeed when hitters are pounding your hard stuff. That’s what’s happened to Danny Duffy in September.