Burnett, 21, was the Packers' choice in the third round of this year's draft. He started all of Green Bay's four games this year and had 14 tackles and an interception.
Packers Coach Mike McCarthy said Burnett's injury was noticed during the second quarter of last week's game but no one knows exactly how it occurred.
Subsequent examinations revealed a torn anterior cruciate ligament in Burnett's left knee. Green Bay placed Burnett on injured reserve Thursday.
Derrick Martin, a fifth-year pro, filled in after Burnett was hurt Sunday and is likely to take the starting safety position Sunday when Green Bay plays at Washington.