Kansas City Royals: Breaking down Danny Duffy’s incredible September

KC Royals, Danny Duffy (Photo by Ed Zurga/Getty Images)
KC Royals, Danny Duffy (Photo by Ed Zurga/Getty Images) /

Danny Duffy has been open to moving to the bullpen but given his performance in September, the Kansas City Royals may be forced to keep him in the rotation.

Up until Danny Duffy headed to the injured list with a strained hamstring, the 30-year old Kansas City Royals starting pitcher was having an underwhelming season with a 4.93 ERA in 100.1 innings of work. He was allowing opponents to bat 0.269 off him and an 0.818 OPS.

Earlier this year, Duffy had indicated that he was open to moving to the bullpen next season if the Royals felt that was best for his longevity. He need not look any further than his teammate Ian Kennedy, who was moved from the starting rotation and has experienced tremendous success as the team’s closer.

"“I talked to them about it,” Duffy said. “I’d love it. I could do it down there. I’ve done well as a starter in my career, but I’ve done well as a reliever. I’ve thrown that idea around.” – MLB"

Duffy has not made an appearance out of the bullpen since 2016 but the numbers are definitely there to support the move. In 34.2 innings, Duffy has a 2.08 ERA (11.4 K/9, 0.26 HR/9) as a reliever with an opponents’ slash line of 0.205/0.271/0.276 and has allowed just one home run. For comparison’s sake, Duffy has a career 4.04 ERA (7.78 K/9, 1.12 HR/9) and an opponents’ slash line of 0.253/0.324/0.413 in 1020.1 innings as a starter.

Statistics actually show that Duffy should have a worse ERA than he actually does a starter considering he has a 4.26 FIP which likely points to some luck being involved. Prior to his return from the injured list, it looked like a no-brainer, until Duffy’s month of September. In all, Duffy tossed 30.1 innings while striking out 25 batters, spinning a 2.37 ERA, and allowing opponents to hit just 0.185 off him. Statistically, it was the best final month of the regular season that Duffy had ever had.

So what ultimately contributed to his success?

It’s actually fairly simple. Duffy rediscovered his changeup in the final month. Per StatCast, he threw 94 changeups in September with nearly all of them coming against right-handed batters, 90 to be exact. And in those at-bats, opponents were able to bat just 0.154 off his changeup and struck out 27 percent of batters with this pitch.

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The only other month he experienced similar success with his changeup was in July when opponents batted a ridiculous 0.091 against that pitch. However, his other pitches were not kind to him as he spun a 4.39 ERA in 26.2 innings and allowed opponents to bat 0.282 combined. Had he used his changeup more in July, he might have experienced similar success. Looking at the numbers, he probably should have as he was able to strike out 33 percent of batters with the pitch but only threw 53 changeups for the month, nearly half of September’s total.

If we look back to April 2016 when Duffy was utilized solely as a bullpen arm, he threw 19 changeups and allowed batters to hit 0.474 off that pitch. It would seem that the success of his changeup is not seen in short outings. Duffy was utilized as a starter in June 2016 and allowed opponents to bat just 0.139 off his changeup while throwing that pitch 94 times.

Certainly, the numbers are there to support moving Duffy to the bullpen in 2020 but he would absolutely have to re-think his approach to getting batters out. The same thing happened for Ian Kennedy when he transitioned from the rotation to the bullpen. In 2018, Kennedy threw his changeup 10.2 percent of the time and that has dramatically decreased below 2 percent this year.

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The difference is that Kennedy was able to maintain the same opponents’ batting average, but could be a coincidence considering that he threw just 19 changeups all year long versus 211 last season.