"When they told me six to eight weeks, my goal was to come back in four," Duffy said.
Duffy's next regularly scheduled start would be Sunday afternoon against Dodgers ace Clayton Kershaw in Los Angeles, although Kansas City manager Ned Yost wouldn't commit to that as of Monday.
For the time being, Yost was just happy to have Duffy back before the All-Star break.
"We don't try to get our mind … wrapped around getting guys back earlier than anticipated." Yost said. "If it happens, it happens, and it did with Danny."
Duffy, who is 4-4 with a 3.54 ERA, tuned up with two rehab starts for Triple-A Omaha, the last of which came Thursday in Colorado Springs. Duffy held the Sky Sox to one run on four hits over 4 2/3 innings with five strikeouts in that game, a 8-4 Omaha loss.
Meanwhile, the Royals on Monday called up right-handed reliever Al Alburquerque from Omaha and optioned left-hander Eric Skoglund to the Triple-A club, while transferring southpaw Matt Strahm (torn patellar tendon in left knee) to the 60-day disabled list.
Alburquerque, who had a 2.08 ERA in 26 innings for Omaha, figures to help a bullpen that's been used a lot in recent days. The Royals have six games in Seattle and Los Angeles this week, with an off-day Thursday.
"He's got a fresh arm, power stuff," Yost said of Alburquerque. "He hasn't thrown in four or five days, and he's a veteran. Right now we'll do whatever we need to do. … We're thin again."