Pino had been extremely effective in relief this season for the Royals, giving up just one run over 14 innings. But in an emergency start for the injured Yordano Ventura on Friday, Pino was rocked for 11 hits and six runs in 5 1/3 innings in the Royals' 7-3 loss to the Red Sox.
Brooks will be used in long relief.
The original plan was to have Duffy get his pitch count to 100 in his rehab stint. Duffy was at about 65 in his last outing in the Minors on Wednesday. But the Royals did not want to risk another shaky start from an emergency starter, and thus will turn to Duffy, who opened the season as the No. 2 starter.
Duffy went on the disabled list on May 25 with a sore left shoulder.
Since going on his rehab assignment, Duffy has altered his delivery, getting his hands higher over his head to try and create a more distinct downhill plane.
"I feel great," he said. "The bullpens have been great. I'm feeling really comfy with my new delivery. Everything is on a downhill [plane]. When I'm missing, I'm missing north and south.
"I just didn't feel anything was ever locked in before. Now I do. Everything is flowing. It's very rhythmic. I feel strong. My shoulder obviously feels better. That's what cortisone and rest does."
Duffy also believes the new delivery will put less stress on his arm and shoulder.
"I think it is a definite possibility," he said. "I was getting too side to side. When you get side to side, you put more stress on your shoulder.
"I think I'm ready in every aspect. I'm excited to get out and do my part. It's been a while since I did my part."
Jeffrey Flanagan is a reporter for MLB.com. Follow him on Twitter at @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.