Posey, a four-time All-Star and the National League's 2012 Most Valuable Player, fouled a Masahiro Tanaka pitch off his back foot in the second inning. Posey proceeded to draw a leadoff walk and jogged to second base after Brandon Crawford singled.
Posey singled to open the fourth inning but was replaced by Trevor Brown when the Giants took the field in the bottom half of the frame.
"It just wasn't feeling really better," Posey said.
Posey, 29, has been durable this season, starting 71 of San Francisco's first 96 games at catcher. Including six starts at first base and four as the designated hitter, Posey is on pace to start 137 games.
Posey, an acknowledged team leader, tried to maintain perspective when asked about the Giants' season-high six-game losing streak.
"The last two games, if a couple of things go our way, we win them," Posey said, "and I think that's the way we have to look at it."