Fox said a magnetic resonance imaging exam revealed Harris suffered a torn anterior cruciate ligament during Sunday's win over San Diego in the AFC divisional playoffs.
The injury means the Broncos will be without a key member of their defense when they meet New England next weekend in the AFC Championship Game.
"Thanks for the prayers, Gonna work extremely hard and come back a Monster next year. God is on my side," Harris said on Twitter.
Harris has compiled 65 tackles and three interceptions this season, stepping up to replace the previously injured Champ Bailey in Denver's secondary.
Notable deaths of 2014 [PHOTOS]