Rivas was optioned to Triple-A Iowa on Friday, Gordon Wittenmyer of NBC Sports Chicago reports.
Rivas held the starting job at first base for Chicago earlier this season, but he'll head to the minors while in the midst of a 1-for-22 slump, during which he's struck out nine times. Overall this year the 25-year-old has a .194/.296/.301 slash line in 38 games.