At the time, Keane had not appeared for United for two months as he was nursing a broken metatarsal in his left foot that he suffered in a 0-0 draw against Liverpool. His exit came soon after he tore into his United team-mates on the club's broadcasting channel MUTV, after they were beaten 4-1 by Middlesbrough.

The footage was deemed to be so severe, it never got brodcast. Nevertheless, his exit was still surprising, as Ferguson suggested not long before that Keane was close to making a return to the pitch. Nearly 20 years later, the Irishman admits he still feels he had a reason to be unhappy with his exit.

While speaking on Stick to Football, brought to you by Sky Bet, Keane said: "I don't see how things could have been different with Sir Alex. It's a silly thing to be thinking about that's not going to happen. I get my anger is obviously a long time ago but the worst anger you can have is the justified anger where you feel - again I feel I hadn't done anything wrong.

"I was 34, I had a broken foot, and my contract was coming up, it was an easy fix for [Manchester] United to go, 'Your days are numbered, alright, cheers, thanks a lot' and I've seen players leave but I could have left in the summer.

"I had no hang ups about leaving in the summer, or going, 'Am I getting another sneaky deal here?' Listen, I'm a big boy, I can deal with it. It was all the way it was done; it wasn't nice that's the bit. Not that I had to leave United, no again, that's life."

Keane is still unhappy with the way he left United (Image: YouTube/The Overlap) His sudden exit brought down the curtain on his 12-and-a-half years at Old Trafford. Keane would eventually join Celtic in the following transfer window, signing an 18-month deal with the Scottish giants.

But he wasn't able to arrive at Celtic Park until January 1, meaning he spent the best part of one and a half months without a club. Looking back, he admits that it may have been more beneficial if he did not leave at the moment he did.

He added: "In hindsight, if I went over to the secretary's office at Manchester United asking that if I signed for another club, do I have to leave today, and they would have said no, then that would have changed my mindset. If I was unable to play for another team till January, then I would have played with the reserves at United.

"Some of the best decisions I've made in my life have been instinctive, and when I left Manchester United, my gut feeling was that the respect had gone for both parties, but the one who was going to suffer the most was me.

"I had bust ups with Sir Alex Ferguson in the past, but for him to say that me leaving the club was the end of my career was harsh from him. I walked out with an injury, which was a double whammy for me as I couldn't play till January.

"When I left the club, there were a lot of stories that came out about me in the press, mainly focusing on my relationship with players, but it was all lies, which can tarnish you, but I can live with it and am at peace with it all."