Queens Park Rangers midfielder Barton made the claim at the Leaders in Sport conference in October, also saying that: "[Ferguson] couldn't even put out the cones. There is a big difference between a coach and a manager."
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"How the hell does he know?," he told Absolute Radio. "Well, let me put it this way: last season he – and I see that as his biggest achievement ever – he won the Barclays Premier League by 13 points.
"I mean, even with the best team in the world, invincible Arsenal, that didn’t happen, but he did that with a team which, this season, are struggling.
"He is the difference. He won the championship with that squad, which for me is not the best squad that Manchester United has had, but if you look at statistics, it's the best squad ever because of that [13-point] gap."
Schmeichel also stood up for the much-maligned Marouane Fellaini, who has struggled for form since joining United for £27.5 million on the final day of the transfer window.
Fellaini has bore the brunt of criticism from the club's fans, but Schmeichel says his hefty price tag has brought extra pressure.
"If we brought Fellaini in for let's say £15m, that's still an awful lot of money, but that kind of pressure wouldn't be on him," he continued.
"I feel sorry for him, he is a good player to have at Manchester United, there's no doubt about that. But because of that transfer fee, it just puts so much pressure on him."