This is a shrewd move by the Steelers. Facing a logjam at OL, the Steelers freed up a roster spot by putting Darnell Stapleton - last year's starter - on IR. With Trai Essex taking that starting gig, this gives the Steelers the chance to keep another young prospect.
I think the OL shapes up like this:
Starters - Kemo, Hartwig, Essex, Starks and Colon
Backups - Legursky (first guy off the bench), Urbik (can't hide him on the practice squad), Foster (should make the team) with Capizzi and Hills battling it out for the 9th spot. Shipley may go to the PS.