"This was a close call," manager Bruce Bochy said before the game. "He's available to pinch-hit, and if something happens we can throw him in the game. We could have done it yesterday."
Posey went through treatment and tests before the game with head athletic trainer Dave Groeschner, and despite noted progress, the Giants decided to sit their All-Star catcher and start Hector Sanchez to catch starter Barry Zito.
"Just talking to [Groeschner] -- he thinks it's the smartest thing to do with 38 games to go," Posey said. "I'm definitely feeling better today."
Posey said the hamstring issue only affects him when he is running and not while he is squatting to catch. He added that he hadn't had a hamstring issue before in his playing career.
"He's not 100 percent, and it's not worth the risk," Bochy said. "We'll give him another day. This is being cautious right now."
Posey echoed that not starting Thursday was out of precaution, and he expects to return Friday.
"I'm hoping tomorrow," Posey said. "I don't see any reason why not."