"He needed the rest," Bochy said. "I could see he needed the rest; I was planning on it."
Posey hasn't sat out an entire game since April 23. He pinch-hit on his last day off.
Bochy called Posey's healed ligaments from his season-ending ankle injury a year ago "a little cranky, but not bad."
Posey was 0-for-8 in the first two games of the series against the D-backs. He is batting .284 with four homers and 12 RBIs on the season.
"I think it's the best thing in the long run," Bochy said. "He needs a day."
Hector Sanchez got the start Sunday with his usual batterymate Barry Zito on the mound. The 22-year-old has hit at a .255 clip with nine RBIs in 18 games this year.
"He's done a nice job. He's makes it easier to put Buster at first or to give him a break," Bochy said. "He's got a good idea of what he's doing out there, he manages well, he has made a lot of progress. Coming out of Spring Training, he was a young guy, but he's done a good job of preparing."