Simmons (shoulder) will begin a rehab assignment at Triple-A Iowa on Thursday, Meghan Montemurro of the Chicago Tribune reports.
Simmons took live batting practice Monday, and he's slated to play five innings and take three at-bats during Thursday's rehab game. The 31-year-old has been on the injured list since July 11, but he'll likely be in the mix to rejoin the Cubs this weekend or early next week.