Johnson replaces Aqib Talib, who was placed on injured reserve after tearing a tendon near his hip bone on Sunday.
Originally signed by the Denver Broncos as an undrafted free agent out of Jackson State in 2009, Johnson was signed off the Broncos' practice squad by the New York Giants last November, and has 10 tackles in 10 NFL games over the past two seasons.
The Giants waived Johnson last month.
Rookie fullback Rendrick Taylor (Clemson) has been signed to the practice squad to replace Johnson.
"I firmly believe that any man’s finest hour, the greatest fulfillment of all that he holds dear, is that moment when he has worked his heart out in a good cause and lies exhausted on the field of battle – victorious."