Riley (ankle) was placed on injured reserve Monday, Teresa Varley of the Steelers' official site reports.
Riley suffered a high-ankle sprain in Sunday's loss to the Browns and was likely looking at a multi-week absence. With his move to IR, the Army product will now be guaranteed to miss at least four weeks before he can return to action. In a corresponding move, Trenton Thompson was signed to the team's 53-man roster after being elevated from the practice squad each of the last two weeks.