Keane made himself into a legend on the Old Trafford terraces for his performances on the pitch during a 12 and a half year association with Ferguson's Reds.
The pair were thought to be extremely close - with Keane supposedly the manager's right-hand man on the pitch. However, the Irishman left United under a cloud in November 2005 after being ushered out the door following an infamous interview with MUTV where he heavily criticised certain players for under-performing.
More recently, the pair exchanged verbal barbs after United exited the UEFA Champions League in the group stage this season. Keane thought he had a right to question the attitude of some of United's current crop of young stars while Ferguson chose to hit back at his former captain's managerial record.
Keane then responded by accusing Ferguson of failing to stand by him following his abrupt exit from the club in 2005.
Asked if he could see a time when he would have a good relationship with the Scot again, Keane said in the Sun: "I wouldn't have thought so, no. He's a busy man and I'm fairly busy.
"I said before I was disappointed the way I was treated at the end, nobody will change that. But that doesn't mean to say I'm bitter and twisted towards Man United. Far from it.
"The worst thing is that when you have been associated with a club you still have loyalty towards it no matter what way I finished and no matter what my relationship with the manager might be.
"I can go back there, I just wouldn't be comfortable so that's my choice."
Keane added: "I just thought the young players needed to buck up their ideas. The boy Phil Jones was speaking before the Basel game saying, ‘I've nothing to prove', and my attitude was every time you walk out on a football pitch you've something to prove.
"I understood where Ferguson was coming from. It was only natural he was going to come out and have a dig back because he's going to defend his young player and he's going to be upset after a defeat.
"When there's snide comments coming in, I had a little dig back. That's the end of that now.
"But you have to defend yourself when people are being critical of my managerial record. I said at the time he should remember the players who helped him get the record he's got - Nicky Butt, those type of lads.
"I remember the lads who did well for me at Sunderland and Ipswich."
Keane will be among the former Manchester United greats who will suit up against Real Madrid legends next month in a charity game organised by the newly-crowned La Liga champions.
The Corazon Classic Match 2012 will take place at Estadio Santiago Bernabeu on Sunday 3 June with a return match at Old Trafford also agreed for next year, on 2 June 2013.
Keane wil join Edwin van der Sar, Denis Irwin, Andy Cole, Dwight Yorke and Bryan Robson while Real's team will include the likes of Zinedine Zidane and Luis Figo among other names.