Posey said he developed blisters along his left side "that are uncomfortable." He said he first saw the blisters Sunday.
Asked if was in pain, Posey, who is on medication, said, "Yeah, just uncomfortable. The blisters are just uncomfortable."
Hector Sanchez replaced Posey in the lineup. Sanchez caught Barry Zito on Monday when Zito threw a four-hit shutout as the Giants won, 7-0, in the Rockies' home opener. Sanchez has become the personal catcher for Zito, so Posey wasn't scheduled to play Monday.
Bochy said he turned to Sanchez on Wednesday with an eye toward having Posey return to the lineup Thursday afternoon when the Giants and Rockies finish their three-game series.
"With the day game tomorrow and being on medication and still feeling quite crummy, that's why he's not playing today," Bochy said.
Shingles results from the same virus that causes chicken pox and manifests itself as a painful skin rash with blisters. Posey said he wasn't worried that any medication would affect his ability to play or that shingles would be anything that lingers and bothers him during the season.
"It's just got to play itself out," Posey said.
Posey has returned from a severe injury that ended his 2011 season May 25. He was hit in a collision at home plate and suffered three torn ligaments in his left ankle and a broken bone in his lower left leg. And now early in the 2012 season, Posey, who has played three games and gone 4-for-12 with a home run and two RBIs, has been sidelined with, of all things, shingles.
"It's just one of those things that happens," Posey said. "It could have happened some time last year, so just look at it like that."