Pruitt said Sunday on "The Nation" radio show that Kongbo tore his ACL on Saturday during the Volunteers' 30-24 victory at Auburn, which was ranked 21st at the time. Auburn fell out of the Top 25 after the loss.
Pruitt said Kongbo would have surgery "in a couple of days."
Kongbo intercepted a pass during Tennessee's upset of Auburn, which snapped the Volunteers' 11-game losing streak in Southeastern Conference games. The fifth-year senior had started five of Tennessee's first six games this season and had 11 tackles.
Tennessee (3-3, 1-2 SEC) hosts No. 1 Alabama (7-0, 4-0) on Saturday.