Danny Duffy made his debut with the Kansas City Royals on May 18th, after going 3-1 with a 3.43 ERA at Omaha and has been off to a modest start with the Royals to the tune of a 2-4 record with a 4.58 ERA. Along with the basic numbers he also has 49 strikeouts against 27 walks. However if you look at what Duffy has done recently you notice he is quietly starting to piece things together at the major league level.
In Duffy's last six starts he has posted just a 1-2 record but has an outstanding mark of 34 strikeouts against just 6 walks. During that time he has pitched into the sixth inning or later in four starts (that includes one AAA start). Over the course of his last six major league starts he has given up more than two earned runs just once, and that was in Colorado, a ballpark that often makes good pitcher look bad.
Though they are obviously different pitchers, for comparison I went back and looked at Zack Greinke's first 11 starts in 2004 as a rookie with the Royals. While Greinke is right-handed and Duffy is a left-hander they are both aggressive hard-throwing pitchers and Duffy is the most highly-touted rookie pitcher on the club since that time. Greinke had a a 4.57 ERA with 44 strikeouts and 13 walks during his initial 11 outings. While it seemed to me that Duffy has given up quite a few home runs so far (10), Greinke allowed 13 home runs in his first 11 games.
Greinke would go on to finish that first season with an ERA of 3.97 (though his xFIP was 4.36). His K/9 rate was 6.21 while his BB/9 rate finished at 1.61. As of the moment Duffy has an xFIP of 4.08, a K/9 rate of 7.47 and a 4.12 BB/9 rate. So Duffy is right where he needs to be at the moment if you look at past history for a rookie, and if he continues to decrease his walks allowed, the numbers will only continue to look better.
Looking even further, Greinke gave up just six infield hits his entire rookie season (24 games), while Duffy has allowed six so far. Over such a small sample thus far, stats like that can negatively effect statistics for a pitch. Duffy certainly has room to improve his numbers, but at first glance he looks like he is well on his way.