In a brutal post-match evaluation in Greece, Keane rubbished the idea the Premier League is 'the best league in the world', while he claimed United manager David Moyes does not have enough 'quality' at his disposal.
United were lacklustre in their UEFA Champions League last 16 first leg on Tuesday with Alejandro Dominguez and Joel Campbell scoring either side of half-time for the hosts.
Keane slammed the standard of English football, claiming Premier League clubs are dropping behind the best teams on the continent.
"I think we're brainwashed by the Premier League," Keane said on British television channel ITV.
"It's not the best league in the world. Nonsense. It's the best brand.
"Our teams have fallen behind the best European teams, United in particular."
Last week, Bayern Munich defeated Arsenal 2-0 in London, while Barcelona trumped Manchester City by the same scoreline at Etihad Stadium, meaning three of England's four teams in the last 16 are behind in the race for a quarter-final spot.
Chelsea are set to face Galatasaray in Istanbul on Wednesday.
The Premier League had no teams in the quarter-finals of the Champions League last season, ending a run of 15 straight seasons where at least one side made the final eight of Europe's premier club competition.
After the loss in Piraeus, United midfielder Michael Carrick claimed it was 'not ideal' and his response sparked another tirade from Keane.
"I want him to say something. Say a bit more - a bit more urgency in his interview - it just reflected United's performance tonight. Flat, no urgency," Keane said.
"They keep saying 'oh well, next game, next game' but for some of them there won't be another game. That's the reality of it."
While Moyes has copped plenty of criticism for United's plight this season, Keane seemed to indicate it is not the former Everton manager's fault.
"There are a number of players who haven't showed enough quality and there is a lack of confidence," the Irishman said.
"United need six top quality players with Vidic, Rio, Evra, Giggsy (all going), he needs to rebuild the club.
"Moyes will be shocked, he would expected good things with this squad, he'll be shocked by lack of quality he's working with."
