Posey, the 2012 NL Most Valuable Player and two-time All-Star, said that he probably will be sidelined for a couple of days "if I had to guess."
"Buster needs a day [off]," manager Bruce Bochy said. "He might need a couple. We'll see how he feels tomorrow."
Posey, 27, said that he hurt himself while taking a "funky swing" as he struck out in the seventh inning of the Giants' 4-1 victory Sunday against Miami. Though he suspected nothing amiss at the time, Posey said that his back stiffened after the game and during Monday's scheduled off-day.
"I really don't think it's anything super serious," said Posey, who said that he'd be available to pinch-hit if necessary.
Posey started 38 of San Francisco's first 45 games, including 33 at catcher and five at first base. He's batting .278 with seven home runs, 23 RBIs and an .813 OPS (on-base plus slugging percentage).
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. This story was not subject to the approval of Major League Baseball or its clubs.