Prieto is expected to be optioned to Triple-A Memphis on Saturday, Derrick Goold of the St. Louis Post-Dispatch reports.
The move will make room on the active roster for Cardinals prospect Joshua Baez, who will make his major-league debut against the Cubs on Saturday. Prieto was brought up from Triple-A on Aug. 4 but has gone 1-for-11 in five games since his promotion to the big leagues. He's been utilized as a depth piece in the infield but will see regular playing time in the minors.