Prior (0-4, 7.71 ERA) was scheduled to start this Sunday against the New York Mets. The Cubs will sub rookie Sean Marshall instead.
Prior, 25, injured his oblique during batting practice last Saturday in Milwaukee. He went home to San Diego for the All-Star break, and at Thursday's workout at Wrigley Field, he was able to play catch.
By moving his start date back, Prior could return within a week. The right-hander was scheduled to throw a light bullpen session on Saturday, and a heavier one on Tuesday, if he can.
"He was better [on Thursday]," Cubs manager Dusty Baker said. "The thing with those injuries are they tend to linger. Every time you're feeling well and you make one wrong move, it's back again. We want to make sure it's gone."
In a corresponding roster move, the Cubs recalled infielder Ryan Theriot from Triple-A Iowa. The Cubs have an off-day on Monday, and can get by with four starters for now. Rookie Carlos Marmol will likely open Chicago's series on Tuesday against Houston.
Prior opened the season on the disabled list with a strained muscle in his right shoulder. For the first time in his career, he has lost four starts.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.