Although the timetable for Riley's return is unclear, such a surgery typically requires four weeks of recovery. Riley exited Sunday's 23-7 loss to New England after playing just seven snaps.
The third-round pick out of LSU started five of Atlanta's first six games at weakside linebacker. His absence could mean more playing time for converted strong safety Kemal Ishmael, who was singled out by coach Dan Quinn for his tackling against the Patriots. The Falcons could also activate LaRoy Reynolds from injured reserve coming off a chest injury.
Reynolds, who saw some first-team reps ahead of Riley during the preseason, returned to practice last week. He has not played in a game this season.
Riley was drafted to bring more speed to the linebacker position. Missed tackles have been an issue for him, but the coaches believe the problem was addressed during the bye week. Riley has 19 combined tackles this season.
The Falcons have had an assortment of injuries on defense, including losing top pass-rusher Vic Beasley Jr. for a couple games because of a hamstring injury, and losing defensive lineman Courtney Upshaw for three games with an ankle/knee injury. Free safety Ricardo Allen missed a game due to a concussion. Defensive tackle Jack Crawford was placed on season-ending injured reserve with a biceps tear.
Beasley, Upshaw, and Allen are back in the lineup now.
The Falcons (3-3) face the New York Jets (3-4) Sunday on the road.