Volstad was sent to Triple-A Iowa after opening the season 0-6 with a 7.46 ERA in eight starts with the Cubs. In eight starts at Iowa, Volstad was 2-3 with a 4.44 ERA, giving up 25 earned runs on 59 hits and 12 walks while striking out 41 over 50 2/3 innings.
"His last outing and even the outing before were pretty positive," Cubs manager Dale Sveum said of Volstad. "It's logical -- it's his day [to pitch] and he's done a good job his last couple outings. It's a logical thing to do."
Dempster, on the DL since June 16, threw on flat ground Saturday.