For years, the Royals had been waiting on Danny Duffy. They waited more than nine years since they selected him in the third round of the 2007 Draft, through a brief retirement in '10, Tommy John surgery in '12, and a career 3.80 ERA with only 352 strikeouts in 443 1/3 innings in parts of five up-and-down seasons entering '16. They waited six weeks into the '16 season to put him back in the rotation from the bullpen, and then only after Christopher Young and Kris Medlen got injured.
By signing a five-year extension worth $65 million (per MLB.com sources), as they did over the weekend, the Royals made a bet that the wait is over. They bet that the Duffy we saw flashes of in 2016 is the one we'll see going forward a whole lot more than the one who was injured or inconsistent for most of his first five big league seasons. So are they right? The good news for Kansas City fans is that it's not hard to see why the Royals think they are.
Let's look at the difference between strikeout rate and walk rate, and see who had the biggest step forward. It boils down to "more strikeouts are good, fewer walks are also good and the difference between the two tells you something about a pitcher." The fact that Duffy leads this list isn't the impressive part. It's that he leads it by twice as much over one of baseball's true aces.
Biggest improvement in difference between strikeout and walk rate, 2015-16
+11.7% -- Duffy, K.C.
+6.7% -- Justin Verlander, DET
+6.1% -- Robbie Ray, ARI
+5.0% -- Julio Teheran, ATL
+4.7% -- Carlos Rodon, CHW
Min. 100 innings both years, 93 qualifiers
Duffy increased his whiff rate from 17.4 percent to 25.7 percent, basically going from "Jimmy Nelson and Colin Rea" to "Chris Sale and Jon Gray." He cut his walk rate from nine percent to 5.8 percent, going from "Edinson Volquez and Shelby Miller" to "Noah Syndergaard and Kyle Hendricks." If that sounds impressive, well, it is.
• Duffy on KC: 'I wouldn't have it any other way'
Among 142 pitchers with 100 innings in 2016, Duffy ranked eighth in lowest in-zone contact allowed, tied with David Price, and he did that despite having the fourth-highest percentage of pitches in the zone in the first place, just behind Clayton Kershaw. (Throwing strikes is good, throwing unhittable strikes is better.) He ranked eighth in highest swinging-strike percentage, just above Corey Kluber and Yu Darvish, and he was second only to James Paxton in fastball velocity from a lefty starter, at 95 mph.

When Duffy struck out 16 Rays on Aug. 1, he tied Kershaw for the most swinging strikes in a single game in the PITCHf/x era, which dates back to 2008 -- and he did it on 22 fewer pitches than Kershaw. When you look at the top five of that list, you'll see a trio of aces in Kershaw, Max Scherzer and Stephen Strasburg; you'll see Duffy; and you'll see Cleveland's Carlos Carrasco, who did it while coming one strike short of a no-hitter in 2015.
We take care to mention Carrasco specifically, because there are some pretty stark similarities between the two, starting with the fact that Duffy himself brought up the Cleveland starter as someone he's watched when speaking to MLB.com in June. How did Duffy improve? In many of the same ways Carrasco did, and remember, when Carrasco has been healthy, he's been elite.
Consider their shared career paths ...
Rotation to surgery to bullpen, and back
Duffy, as we noted above, struggled to stick in the rotation for years, injured his elbow, returned for more inconsistent work in the rotation and started his "breakout season" in the bullpen, five years after his debut, at age 27. Carrasco's path took roughly the same shape, because after being acquired in the 2009 Cliff Lee trade, he spent three years unable to stick with the Indians, missed '12 after Tommy John surgery, was inconsistent after that and spent a big chunk of his '14 "breakout season" in the bullpen … five years after his debut, at 27.
All stretch, all the time
The move to the bullpen wasn't just a chance to take a breather from going deep into games, it was a chance to completely change how Duffy (and Carrasco) pitched. Remember when we said that Duffy cut his walk rate and began throwing a ton of strikes? That doesn't just happen by itself. Like most relievers, Duffy worked exclusively out of the stretch, but he kept doing it when he got back to the rotation, eliminating the windup.
"This year, the stretch has helped me immensely with my command," he told MLB.com. "There's no shame in that, and I don't know if I'll ever throw another pitch out of the windup, to be honest with you. It's very simplified, it's very nice. Clean, simple delivery."
Duffy mentioned Carrasco as a model, because he did exactly the same thing in 2014, allowing for simplified mechanics and better control.
Fewer fastballs, more breaking pitches
Despite the fact that both pitchers throw hard, they both found more success when they offered the fastball less. When Carrasco returned from the bullpen in 2014, he threw his 96-mph fastball less often, in favor of his slider and a deadly pitch labeled as a changeup that catcher Yan Gomes once referred to as a "changeup/split, whatever he wants to call it."
As a starter in years past, Duffy would routinely throw his four-seamer about 60 percent of the time, a rate he continued in his first month out of the bullpen in 2016. But then he began to gain confidence in his new slider, a harder offspeed pitch he'd begun working on the previous fall to replace his slower, less effective curveball. It looks like this, and it is very hard to hit:
Gif: Danny Duffy strikeout
After returning to the rotation, the fastball appeared only 36 percent of the time, and both pitches were more effective, since there was far less visual separation than there had been with the slower curve.
We could go on; Carrasco (25 percent K rate, 5.7 percent walk rate) and Duffy (25.7 percent K rate, 5.8 percent walk rate) even had nearly identical whiff and walk rates in 2016. If Duffy works out as well as Carrasco has, the Royals will be thrilled -- and so will Duffy, who passed up a chance to be the third-best starter behind Darvish and Jacob Arrieta in free agency next offseason in order to stay where he's comfortable.