"Ventura spent the night throwing up [from the flu], so he'll stay here," Yost said.
Volquez, who could be the Opening Day starter, has hit the dead-arm stage of Spring Training.
"His arm is heavy, a little fatigue," Yost said. "We're going to back him off one start, just have a side session."
Right-hander Dillon Gee, just added to the 40-man roster, will make the start on Friday in San Antonio. Chien-Ming Wang, one of the surprises of camp, will get the start on Saturday.
Meanwhile, in the split-squad games in Surprise, right-hander Miguel Almonte will start on Friday and Matt Strahm on Saturday.