The Royals outrighted Diaz to Triple-A Omaha on Tuesday.
Diaz will have the ability to elect free agency, but assuming he accepts his outright assignment to the minors, he'll remain part of the Royals organization as catching depth. The 35-year-old backstop recently wrapped up a month-long stint on the big-league roster, appearing in 10 games while batting .227 with two home runs and five RBI.