The Giants optioned Diaz to Triple-A Sacramento on Saturday, Maria I. Guardado of MLB.com reports.
Thairo Estrada's return from a hand injury will bump Diaz from the active roster. Once a highly touted prospect, Diaz fizzled out in Miami and has gone just 1-for-19 across two stints with San Francisco this season. He could be a DFA candidate later this year.