Derrick Johnson suffered a ruptured left Achilles in Thursday night's win over the Oakland Raiders.
The Kansas City Chiefs linebacker had to be helped off the field without putting any weight on his foot.
"A sad deal there," Andy Reid said. "I felt a little bit of a letdown when it happened."
Johnson, who missed most of the 2014 season because of a torn right Achilles, was injured on Oakland's only touchdown drive of the game.
Johnson is the longest-tenured Chiefs player, along with punter Dustin Colquitt. He arrived in Kansas City as its first-round draft pick in 2005.