Southward struggled after moving from safety to cornerback and was active in only three games. King, a seventh-round pick this year, has been on the practice squad.
The Falcons (6-2) will be without nickel cornerback Robert Alford for Sunday's game at San Francisco (2-6) with a groin injury. Starting left cornerback Desmond Trufant is listed as probable with a back injury.
Rookie defensive end Vic Beasley missed practice Friday with a strained foot and is listed as questionable.
AP NFL websites: www.pro32.ap.org and http://twitter.com/AP-NFL