The Giants had sent down reliever Eric Hacker after Saturday's game and waited until Sunday to announce the corresponding move.
"We just wanted some coverage on the catching side if we get in a situation with Buster not being 100 percent," manager Bruce Bochy said. "We can run for him and still have another catcher."
Whiteside competed with Sanchez for the backup catcher roster spot in Spring Training and joined the club when Sanchez was on the disabled list for two weeks with a sprained knee last month.
Posey started at first base Sunday with Sanchez behind the plate for starter Tim Lincecum.
"Today, for example, [Posey]'s at first base, and if I want to take him out and put [Brandon] Belt out there for his defense, I'll still have another catcher," Bochy said.
Bochy said Posey is "real close" to being back to 100 percent from his tight right hamstring.
"He feels it running just a little bit," Bochy said. "He's taking it easy, but if we get in a situation where we need some speed out there, we don't want him to push it, and having another catcher will allow us to make that move."