The Cardinals optioned Prieto to Triple-A Memphis on Saturday, Derrick Goold of the St. Louis Post-Dispatch reports.
Prieto has struggled at the plate this spring, going just 2-for-17 over six games in the Grapefruit League. With his quest for an Opening Day roster spot now over, the 26-year-old infielder will aim to get back on track in Memphis, where he slashed .301/.363/.457 in 516 plate appearances last season.