Ferguson said in his autobiography that Keane, who spent 12-and-a-half years at the heart of a fearsome Manchester United midfield, struggled to cope when his powers as a player waned.
New Republic of Ireland assistant coach Keane said: "The beauty of football is that everybody has opinions and I’ve no problem with that.
"The issue I have is when anybody who seems to talk about me, or has issues with what I’ve said in the past, and people will just tell lies about me, that’s when I’ll come out and defend myself.
"I’m not going to sit here and defend myself regarding Alex Ferguson. That’s for another day. I’m not really going to get into that too much. Football is about opinions, no problem about that, but when it’s lies, that’s when you come out and say something.
"Today is not for that. Today is about talking about getting back involved with the Irish team with Martin, all positive. It's not an area I want to go at today."
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