"It went really well," Duffy said. "I'm really happy with where my effort was and I was surprised at how under control I was. It felt really good. It was really exciting after a whole year of not pitching."
Duffy, who underwent surgery last June, will rest about three days before throwing again.
Duffy and fellow starter Felipe Paulino, who had a similar surgery, are expected to be ready around midseason.
"You cannot get enough starting pitching because, over the course of a year, something is going to happen and you're always going to need it," manager Ned Yost said. "What Duffy and Paulino are is just great insurance policies that are a benefit to us around the All-Star break. But it's nothing to worry about right now. Trust me. I am extremely pleased that we have them both out there and they'll be available around the All-Star break."