Volquez had to leave a game Saturday in San Diego because of spasms. The back bothered him when he tried to throw in the bullpen on Wednesday, and he recommended to manager Dusty Baker that he skip his next scheduled start to give it more rest.
"I threw a couple of pitches, and it was stiff on the left side a little bit," Volquez said, after cutting his session short.
The sore back is a concern for the Reds, who have relied on their pitching staff to make up for their injury-depleted offense. Third baseman Edwin Encarnacion is sidelined indefinitely with a chipped bone in his left wrist, and first baseman Joey Votto -- the team's top hitter -- was examined again on Wednesday to find an explanation for his dizzy spells.
Volquez came over from Texas in the trade for Josh Hamilton and won 17 games last season, when he was the Reds' only All-Star representative. He has struggled with his control this season, walking a team-high 31 batters and hitting five. He is 4-2 with a 4.25 earned run average.
He winced after throwing a pitch to Nick Hundley in the fifth inning of a 6-5, 16-inning loss to the Padres. He went behind the mound and bent forward, trying to stretch the back muscles that had cramped. The injury forced him out of the game.
Volquez said the right side of his back felt much better on Wednesday, but the left side still hurt when he threw. He went into Baker's office, told him of the result and recommended skipping his start scheduled for Saturday against Cleveland.
"It's better to miss a start," Volquez said. "I don't want to go out there at 70 percent and hurt the team."
Baker had already moved his next start from Friday to Saturday to give him an extra day to recover. Volquez will have a medical test Thursday, and the Reds will decide what to do for his next start. They could go with right-hander Ramon Ramirez, who was called up from Triple-A Louisville on Tuesday to replaced injured long reliever Nick Masset.
Ramirez made three starts for Louisville before his promotion, going 0-3 with a 5.08 ERA.
Although the Reds' starting infield has been decimated by injury -- all four were sidelined by injuries or the flu at one point -- their rotation has been healthy until now. Cincinnati leads the NL with six shutouts, matching its total for last season.
Right-hander Bronson Arroyo had another bout of carpal tunnel syndrome during spring training, but has pitched through it and gone 5-3.