Manager Bruce Bochy said Posey contracted a painful rash Sunday night under his left armpit, on his left side and his back. The 2010 NL Rookie of the Year was removed from the lineup for Wednesday night's game at Colorado, with backup Hector Sanchez getting his second straight start.
"It's been getting better every day. If we were in September he could go out there and play but with the day game tomorrow I thought let Hector have this evening and let him play tomorrow," Bochy said. "With the shingles I don't know if he could go both games. I guess it's quite painful and I changed the lineup."
Posey was available to hit, Bochy said.
Brought on by the same virus that causes chickenpox, shingles is a blistering skin rash most common in older people. It usually ends after four weeks, but one in five victims develop long-term nerve pain. Other complications include scarring and loss of vision or hearing.
Last season, former Cardinals manager Tony La Russa was treated for shingles, which left his face swollen and his right eye nearly shut.
Bochy said Posey was on medication, another reason he didn't want to put him behind the plate Wednesday night.
"With the day game tomorrow and being on medication and still feeling pretty crummy, that's why he's not playing," Bochy said. "He's (hurting) one the left side, on the armpit, on the back, on the left side. It's a rash that looks like chickenpox."
Posey played in all three of the Giants' games at Arizona over the weekend but got the day off Monday at Coors Field, part of a plan to ease him back after the brutal left leg injury he sustained in a home plate collision last May 25 with Florida's Scott Cousins.