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The 37-year-old Scotsman came through the United youth academy and broke into the senior team in 2003, two years before Keane would leave the Red Devils.
But Fletcher revealed that the club legend was one of his three biggest influences and how fulfilling it was to receive a compliment from him.
"My dad, Sir Alex Ferguson and Roy Keane are the three biggest influences on my career," he told UTD Podcast.
"They are the people who helped me. The great thing about Roy Keane is that he says ten things to you.
"One of them is classic Roy Keane where he has a go at you, lets you know and is really harsh on you.
"The other nine times, he gives you a compliment, but nobody hears about that or wants to remember that because it is not a good story to tell. But that is the way I remember Roy Keane.
"Roy Keane was fantastic with me. Walking off the pitch beside Roy Keane and him saying to you, 'fantastic today son, I could play for another ten years with you beside me in midfield.'
"Nobody hears that and it made me feel like a million dollars.
"The confidence I got from those words from Roy Keane massively outweighed the handful of times when he had a go at me and rightly so, because the levels or my standards were not that of a Manchester United player coming through, which is completely normal."
Former United captain Keane, 50, became a pivotal figure under legendary former United boss Sir Alex Ferguson and collected a plethora of silverware during his 12 years at Old Trafford.
But the outspoken Irishman caused controversy at the Red Devils when he delivered an infamous interview on MUTV after Middlesbrough's 4-1 thrashing of United in 2005.
Keane slated former teammates John O'Shea, Alan Smith, Kieran Richardson and Fletcher for their performances.
And he also savaged fellow club legend Rio Ferdinand when he said: "Just because you are paid £120,000-a-week and play well for 20 minutes against Tottenham, you think you are a superstar."
Despite being targeted by Keane, Fletcher has no hard feelings towards his brutal assessment and felt that much of his opinions were misconstrued.
"Well, first things first, the headlines that came out of it," he said.
"Roy Keane watched the game and gave an analyst's view of it, so some of the words he used were in reference to an action in the game.
"So, for example, if he was talking about somebody's defending in the moment then he called it 'lazy defending' and the headline in the paper was: 'Roy Keane calls that player lazy,' so I think it a lot of it had been misconstrued."
Fletcher also admitted that he would use Keane's criticism to help him develop his United career and remain motivated.
"For me, I saw it as him saying about instances in the game, in terms of it not being good enough for me, and he was completely right," he explained.
"It was Roy Keane challenging me as Manchester United captain and it was something I was used to in the dressing room.
"The long and short of it is that I knew Roy Keane liked me and that was his way of showing he liked me, because he knew I was capable of more and wanted to challenge me.
"He would say 'yes, I was little bit harsh there on Fletch but it's because I like him and want him to do well.'
"So, for me, it was no problem at all. But I can see why Sir Alex Ferguson had a problem with the interview."
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