In his new book, Ferguson described his former midfield talisman as an 'intimidating, ferocious individual' and claimed he had 'the most savage tongue you can imagine. He can debilitate the most confident person in the world in seconds.'
Keane left United in November 2005, just a few weeks after incurring the wrath of his manager for criticising team-mates in an interview for MUTV.
"I'm OK, quite relaxed," Keane told ITV Sport. "I remember having conversations about loyalty when I was at the club and I don't think he knows the meaning of the word.
"To constantly criticise other players that brought him success ... I won't be losing any sleep over it.
"I just don't think the manager needs to do it. I don't know how many books he's written now.
"A lot of these players helped win a lot of trophies. It's just part of modern life. People like to do books and criticise ex-players."