"He thought he was getting better, but it's actually gotten worse," Bochy said. "That's why we had to scratch him."
The Giants summoned Matt Cain from the bullpen to make the spot start for Bumgarner, who was in line to make his 10th start since rejoining the rotation after the All-Star break. The 28-year-old was set to face the Cardinals for the first time since the 2014 National League Championship Series, when he helped the Giants to a 6-3 triumph with an eight-inning performance.
Bochy announced after Wednesday's loss to the Padres that the Giants will utilize a six-man rotation -- at least for the time being. Johnny Cueto is due to start Friday, followed by Chris Stratton and Jeff Samardzija to round out the series against St. Louis.
But Samardzija, coming off his first shutout as a Giant, could get bumped should Bumgarner feel OK for the series finale.
"We'll wait until we feel he's strong enough to go out there and pitch," Bochy said of Bumgarner. "It could be as early as Sunday, somewhere in there. We just don't know yet."
• Buster Posey was back in the Giants' Thursday lineup and behind the plate after missing three consecutive games with a bone bruise and inflammation in his left thumb. Bochy said rest helped Posey return to the lineup.
• Rookie outfielder Austin Slater will begin a brief rehab assignment Friday night playing five innings for Triple-A Sacramento. Because Sacramento's season concludes Monday, the Giants will likely activate the outfielder and give him the chance to recover at the Major League level -- a luxury afforded by the Sept. 1 roster expansion.
"I don't know if he will be ready or not; we'll see how it goes these next three or four games," Bochy said.