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United were one of the best teams in the world under Ferguson and for a big chunk of those glory years, Keane was Red Devils skipper.
Quinton Fortune joined in 1999 when United had just won the treble and standards were still incredibly high.
He told Ladbrokes at the launch of their 5-A-Side Bet on Brighton vs United that players would get to training two hours before it started.
Fortune explained per Daily Star: "He [Keane] set the standards every single day in training. He had so much influence in the team, and if you dropped your standards, you would hear from him. He wasn't just keeping an eye on whose standards were dropping, though, he was setting them himself.
"He could talk the talk and walk the walk; he was just relentless every day. I wish people could see how much this guy trained; he was unbelievable."
Fortune would remain at Old Trafford until 2006 meaning he saw the exit of United's treble winners and the arrivals of the next generation, most notably Wayne Rooney and Cristiano Ronaldo.
Did Keane expect less from the youngsters, let them slack off? Absolutely not.
They quickly caught on to the rule and would regularly turn up when the seasoned pros did.
Fortune continued: "It was just an unspoken rule; you just picked it up from day one. You needed to be on your toes every day in training, because if you weren't someone else would take your place. It was as simple as that.
"If training started at 10:15am, you'd be thinking 'OK, I need to be in at 9, maybe even 8am'. You'd get in early, have your breakfast, do your stretches, be out on the pitch at 10am, ready for training.
"You saw Gary and Phil Neville doing it, Cristiano [Ronaldo], Roy Keane, all these guys were doing it, so new players would just pick it up naturally.
"We would treat every training session like it was our last, and that became a habit for everyone at the club. You came in and just thought 'is this normal?' but match day would come around and it would be easy."
Keane left United in November 2005 after an explosive falling out with Ferguson and their relationship never recovered.
The Irishman was often Ferguson's voice on the pitch but their similarities meant they clashed off it.
Fortune departed for Bolton Wanderers the following year.
United are currently sixth in the Premier League table and are likely miss out on Champions League football next season.
Erik ten Hag will become their new permanent manager in the summer and he'll be tasked with helping to rebuild the squad.
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