Contreras (hamstring) will begin a rehab assignment at Double-A Portland on Friday, Tim Healey of The Boston Globe reports.
Contreras took batting practice on the field before Tuesday's game against the Angels as the latest step in his recovery from a left hamstring strain. He'd been a candidate to return from the injured list for Friday's series opener against the Royals, but the Sox will instead have him play in a few tune-up games before adding him back to the lineup.