O’Neill, a lifelong United fan, is spearheading the Red Knights group of City financiers who are plotting a crusade to oust the American owners in an estimated £1bn takeover.
O’Neill angered the Glazers after criticising their leadership back in January in the wake of the bond issue.
He said his comments were made in a“personal capacity” but with the Red Knights stepping up their takeover bid this week, the Glazers have decided to act.
The move was revealed on the day when United chief executive David Gill hit back at the Red Knights, questioning the financiers’ credibility and leadership.
The group, which says it is concerned about the £716m of debt amassed by the Glazers, intends to raise money for a bid if enough disaffected United fans pledge their support for a takeover.
But Gill believes that could mean dozens of people having a large say in the running of the club – a system he does not think would bring success.
“Having 20, 30 or 40 very wealthy people running Manchester United – I don’t know how it would work,” Gill said.
“At the better-run clubs there is clear single decision-making and it’s quick and efficient: Roman Abramovich at Chelsea, Sheikh Mansour at Manchester City, Silvio Berlusconi at AC Milan.”
Gill also mocked the group’s credentials, notably those of football takeover specialist Keith Harris, chairman of investment bank Seymour Pierce and one of the group’s most prominent members.
“Keith Harris will go anywhere there is publicity around, we know that and we accept that, but if you looks at his track record in football I don’t think it’s anything to write home about.”