Duffy gave up 10 hits, walked two and struck out two. He left trailing only 4-3 with two runners on and two out.
"I felt better today than I normally do," Duffy said. "Obviously it took a lot more preparation to get to this point. I felt like I had good stuff -- had a really good fastball and threw some good sliders.
"I signed up for this three days' rest stuff. It's what I wanted to do. I wanted to lead. It just didn't turn out the way I wanted it to."
Royals manager Ned Yost also dismissed the notion that the short rest affected Duffy.
"Absolutely unrelated," Yost said. "He felt great. He went 100 pitches. There's days where -- pitchers struggle to have their curveballs some days or command of their fastballs some days. Some days, it's command of their changeup. He just couldn't get a feel for his changeup. It's got nothing to do with the rest."
But Tigers first baseman Miguel Cabrera said Detroit's lineup took Duffy's short rest into account: "That's why I think we did a very good job today taking some pitches, trying to go deep in the count and make him throw a mistake. You have to make sure to hit it."
The Royals' struggling bullpen had issues again, as rookie left-hander Tim Hill walked in two runs. And later, right-hander Blaine Boyer gave up one run, Justin Grimm allowed three more to score in the seventh and Burch Smith surrendered two runs in the eighth after loading the bases with no outs.
Abraham Almonte had three hits and scored two runs for the Royals, who lost for the 10th time in 11 games.
"We're four and whatever," Duffy said. "We haven't played 20 games and we're 10 games under .500. It's just not a good start for us. You can be discouraged or frustrated, but the bottom line is we're going to have to be the ones to pull ourselves out of this."
MOMENTS THAT MATTERED Trailing 6-3, the Royals had a great chance to sneak back in it when Almonte singled and Alcides Escobar doubled to open the sixth. But Drew Butera popped out, and after an RBI groundout by Jon Jay, Whit Merrifield flied out to short right field.
"We got 11 hits," Yost said. "It's like [hitting coach] Terry [Bradshaw] keeps preaching to them -- we're getting guys on. We're not driving guys in, but we're getting guys on. Keep getting on until we start driving guys in. I mean, four runs for us has always been a tell-tale number for our team in the past. We just couldn't hold the opposition past four."
YOU GOTTA SEE THIS Royals left fielder Jay robbed Leonys Martin of a hit in the bottom of the fourth with an excellent running catch near the line. Statcast™ determined it was a four-star catch -- Jay needed to cover 39 feet in 3.1 seconds to make the play for a 38-percent catch probability.
HE SAID IT "Extremely frustrated. This isn't fun. We got to do better." -- Duffy, on the Royals' 4-15 start
UP NEXT Left-hander Eric Skoglund (0-2, 9.31 ERA) takes the mound for the Royals in the series finale in Detroit on Sunday at 12:10 p.m. CT. Skoglund gave up five runs and eight hits over five innings in his last start Tuesday, an 11-3 loss in Toronto. In his one previous start against the Tigers, Skoglund allowed two hits over 6 1/3 shutout innings in a 1-0 win last season in his Major League debut. The Tigers will counter with left-hander Francisco Liriano.