Dempster started last Monday in Miami, and during his bullpen session two days later, he felt some discomfort.
"It was tight and I threw a pitch and it grabbed on me, and I felt it go a little bit," Dempster said. "Maybe I did something during the game, but when you're out there pitching and the adrenaline is going, you don't feel anything. The next day, it was a little tight and I tried to throw my bullpen and it didn't go too well."
Dempster's expected to miss only two starts because of the strained right quad. Randy Wells was called up from Triple-A Iowa on Sunday to take Dempster's spot in the series finale against the Reds. Wells was 1-0 with a 9.42 ERA in three starts with the Minor League team.