The Cubs optioned Assad to Triple-A Iowa on Sunday, Jesse Rogers of ESPN.com reports.
Assad lost his spot in the rotation in the aftermath of the Aug. 3 trade deadline and shifted to the bullpen, and he'll now end up losing his spot on the roster entirely after the Cubs reinstated Edward Cabrera (hamstring) from the 15-day injured list Sunday. Assad is expected to get stretched back out at Triple-A to offer insurance in case injuries test the big club's starting depth down the stretch.