The Falcons placed Harrison (knee) on IR on Saturday, Tori McElhaney of the team's official site reports.
Harrison first hurt his knee in mid-October. He missed two games before returning to action in Week 10 against Indianapolis. However, Harrison sat again during Week 11 and had already been deemed out for this Sunday before being moved to IR. Atlanta coach Raheem Morris said Saturday that Harrison had a setback with his knee and will need to get the issue "fixed," which may mean the third-year pro is going to need surgery.