The pair fell out in 2005 when Keane openly criticised some of his teammates during an interview with the club's television channel MUTV.
Keane had his contract at Old Trafford cancelled soon after and Ferguson has admitted that the argument that preceded his departure was "frightening".
"The hardest part of Roy's body is his tongue. He has the most savage tongue you can imagine. He can debilitate the most confident person in the world in seconds with that tongue," Ferguson wrote in his new autobiography.
"What I noticed about him that day as I was arguing with him was that his eyes started to narrow, almost to wee black beads. It was frightening to watch. And I'm from Glasgow."
Keane originally joined United in the summer of 1993 from Nottingham Forest.