Horwitz (hamstring) will begin a rehab assignment with Single-A Bradenton on Tuesday, Colin Beazley of the Pittsburgh Post-Gazette reports.
Horwitz has been on the 10-day injured list since suffering a left hamstring strain June 24 against Seattle. However, his rehab has been trending in the right direction, and he'll take the next step Tuesday. Horwitz has missed nearly a month of action, so he figures to need at least a handful of at-bats in the minors before being activated.