The 34-year-old Posey hasn't played since suffering the thumb injury during the sixth inning of the Giants' 5-2 win over the Arizona Diamondbacks on the Fourth of July.
He sustained the injury after taking an awkward shot to his glove on a foul tip by the Diamondbacks' Daulton Varsho. Posey underwent an X-ray on the thumb that came back negative at the time, but an MRI on Thursday revealed a bone bruise.
"I imagine being back there catching balls that are moving all over the place, one additional ball caught the wrong way can be a setback, so I think this is the cautious, the prudent and the right approach to get him ready for the second half."
Posey, the National League MVP in 2012, was voted to start at catcher for the NL in next week's All-Star Game. Due to the injury, he said he won't make the trip to Coors Field in Denver.
"Obviously, I'm disappointed," said Posey, who has now been selected to seven MLB All-Star teams. "It's an honor to play in the game. I know my oldest two [kids] were excited about being there, but these things happen and you move on."
Posey has a .328 batting average with 12 home runs and 28 RBIs over 58 games this season for the Giants, who entered Friday with the best record in the NL. San Francisco opens its second-half schedule July 16 against the St. Louis Cardinals.
The Giants said Posey's placement on the IL was retroactive to Tuesday. In a corresponding roster move, the team activated right-hander Logan Webb.