The Cardinals optioned Prieto to Triple-A Memphis on Saturday, Jeff Jones of the Belleville News-Democrat reports.
Prieto boasts an .890 OPS across 300 plate appearances in the minors this season, but he's gone just 1-for-31 at the plate with the parent club. He'll now return to Memphis to rediscover his swing, clearing room on the active roster for top prospect Joshua Baez.