Rios was outrighted to Triple-A Iowa on Sunday, Meghan Montemurro of the Chicago Tribune reports.
Rios was optioned to Iowa after being reinstated from the injured list in mid-June, and he's now lost his spot on Chicago's 40-man roster. The 29-year-old has an .810 OPS at Triple-A this season but struggled to a .071/.235/.214 slash line over 34 plate appearances in the majors.