Votto, who was elected by fans to the National League All-Star team Sunday, has inflammation in his left knee and exited Saturday's game in the fifth inning after having trouble running out a ground ball. He originally injured the knee sliding into third base during Friday's game.
Manager Dusty Baker did not put Votto in the lineup before he knew how he might be feeling. As to whether Votto could play at all, the 28-year-old deferred to others.
"It's progressing. I will have to talk to Paul [Lessard, the head athletic trainer] and Dusty about it," Votto said. "It's usually their decision and it's my knee's decision, not really mine."
Miguel Cairo started in Votto's place and batted seventh. Second baseman Brandon Phillips moved up from the fourth spot to Votto's customary third spot.
Votto is batting .350 with 13 home runs and 47 RBIs this season. His 33 doubles and .471 on-base percentage lead the Majors and his 60 walks are tops in the NL.
Last season, Votto played 161 out of 162 games and he appeared in all 77 of the Reds' previous games this season. He did not start May 30 at Pittsburgh when he was given a rest, but pinch-hit in the ninth inning.
The Reds also had a late scratch in third baseman Scott Rolen, who was replaced in the bottom of the first inning by Todd Frazier after experiencing back spasms during Zack Cozart's leadoff at-bat.
Rolen is batting .184 and missed 34 games this season with a strained left shoulder. He returned from the disabled list on June 18.
"His back just locked up on him," Baker said. "It was just like a few minutes before the game. At that point, we went to Frazier, who had a good day for us."
Frazier finished a 4-3 Reds loss to the Giants 2-for-3 with a walk. His two-run home run to left field off Ryan Vogelsong gave his team a 2-1 lead. Frazier also made a nice diving stop toward the line to rob Buster Posey of an extra-base hit in the first inning.