Zito was already on the disabled list with a sprained right foot that has hampered him since April. The latest injury occurred on a fielding play Saturday night, and he was helped off the field by trainers.
"I saw a picture of the ankle, and it's pretty bad," Giants manager Bruce Bochy said Sunday. "It's really swollen. It looks like there's a baseball in there, so this looks like it's going to be quite awhile."
Zito is 3-4 in nine starts for the Giants this season with an ERA of 5.62.