Riley suffered a meniscus tear, the full extent of which is unknown. A trim of the meniscus typically involves three to six weeks of recovery, while a total repair requires three months of rehab.
Riley will miss at least this Sunday's road game against the New York Jets along with next week's first NFC South clash against the Carolina Panthers in Charlotte.
Kemal Ishmael immediately replaced Riley at weakside linebacker during last Sunday's loss to the New England Patriots. LaRoy Reynolds, who was placed on injured reserve with a chest injury, returned to practice last week and could be activated soon. Reynolds ran ahead of Riley during preseason practice before Riley earned the starting job in the base package.
The Falcons added depth Tuesday by bringing back veteran Sean Weatherspoon, the team's first-round pick in 2010. Weatherspoon was with the Falcons last season but suffered a torn Achilles, limiting him to four games.
Weatherspoon worked out for the team Monday.
"He really looked fit, really looked strong," Quinn told The Game. "With Duke being injured -- and we'll probably be without him for a couple weeks -- we wanted to see where Spoon was at. He's got fantastic leadership. He'll be certainly a part of the group. And as it goes for this week, we'll see where he's at. We'll kind of play it by ear in terms of his readiness and how long it would take for him to get ready and help in some regard."