The former player says his longtime manager has a "massive ego," and also acknowledged he cried following the swift end to his career at Old Trafford in 2005.
The 71-year-old called time on his 26-year reign at Old Trafford at the end of last season but Keane, who featured under Ferguson from 1993 to 2005, believes his former manager is still looking to exert his authority at the club.
"Everything is about control and power," Keane said in ITV4's documentary "Keane and Vieira: Best of Enemies." "He's still striving for it now even though he's not manager. There's massive ego involved in that."
Keane also talked about his departure from United, saying he cried when he left in November 2005 and believes he could have remained at the club for a few more seasons.
"Of course I was upset," he said. "I did shed a few tears in my car for about two minutes. But I also told myself I had to get on with my life. I walked out with nothing, I had no club lined up and I was injured. I told David Gill I had been injured playing for Manchester United. I could have played for Manchester United easily for another couple of years."
Keane, who was appointed as Martin O’Neill’s assistant with the Republic of Ireland national side, says his relationship with Ferguson is now "nonexistent."
The former midfielder also hit out after Ferguson praised Keane in his latest autobiography for "covering every blade of grass" in the Champions League semifinal second leg against Juventus in 1999.
"Stuff like that almost insults me," Keane said. "I get offended when people give quotes like that about me. It's like praising the postman for delivering letters."