Diaz was recalled from Triple-A Memphis on Monday to take Tommy Pham's (oblique) spot on the roster, MLB.com reports.
Diaz will push Jedd Gyorko for playing time at shortstop. The Cardinals signed Diaz to a four-year, $8 million deal in 2014, so he'll likely get some chances to show what he can do while Ruben Tejada and Jhonny Peralta are on the disabled list.