One of Roy Keane's most admirable qualities is that he will not have spent all morning Googling and hashtag-searching his own name on Twitter to gauge the level of hostility he has provoked among certain Manchester United fans for "crimes" seemingly ranging from treason to apostasy. He simply couldn't care.

On Tuesday night, after his former club's defeat by Real Madrid, he said in his role as an ITV pundit that the decision of the referee, Cuneyt Cakir, to send Nani off was "the right call", an opinion that has inflamed some of the millions who disagree with him and their advice over the past few hours has included proposals that he perform an anatomically impossible act, calls for his sacking by the broadcaster and suggestions on how to spend his "30 pieces of silver".

The oddest thing about the abuse is this sense of betrayal. Some supporters seem to feel genuinely let down by Keane as if his loyalty had somehow become perverted by malice. But how can anyone who has any knowledge of his outspokenness as a player – in 2000 after booing during a Champions League victory over Dynamo Kyiv he said of United's corporate fans "they have a few drinks and probably the prawn sandwiches, I don't think some of the people who come to Old Trafford can spell football, never mind understand it", his anger that United linked the rise in season ticket prices in the summer of 2000 to the cost of his new contract, his view that Jaap Stam's transfer to Lazio in 2002 signified football clubs treating players as "pieces of meat" and Saipan – think that he would feel a debt to the club that would curtail his right to be fearlessly frank on television? That he would silence himself?

After all he was even more fearlessly frank and furious on MUTV in October 2005, following Manchester United's 4-1 defeat by Middlesbrough when he was interviewed on the channel's Play the Pundit programme. First he described Kieran Richardson as a "lazy defender", questioned why "people in Scotland rave about Darren Fletcher" and said of Rio Ferdinand: "Just because you are paid £120,000-a-week and play well for 20 minutes against Tottenham, you think you are a superstar." The programme was pulled and the issue became a huge source of embarrassment to the club and particularly Sir Alex Ferguson who decided the best way forward for club and player was to terminate Keane's contract. "I was disappointed the way I was treated at the end, nobody will change that," Keane said last year. "But that doesn't mean to say I'm bitter and twisted towards Man Utd. Far from it."

That, obviously, is not how his critics see it. The very qualities Manchester United fans used to praise – his obdurateness, intolerance for slackness in deed and thought, his perception of himself as a Cork Hercules cleaning up the game's bullshit merchants, his feistiness and lack of sentimentality – are the very ones some now claim to have warped all notions of his duty to the club.

All he said was that he disagreed with the received wisdom that Cakir's interpretation of Nani's collision with Alvaro Arbeloa was wrong. That Nani's boot hit the Real Madrid full-back is a fact. The rest is a debate about the referee's judgment and in Keane's view: "I think the referee has actually made the right call. Everyone's upset about it and it's slightly unlucky, but it's dangerous play. Whether he meant it or not is irrelevant. It was dangerous play – it's a red card. You have to be aware of other players on the pitch. Does he think he's going to have 20 yards to himself?"

When ITV first began the televised football revolution at the 1970 World Cup, Malcolm Allison, Pat Crerand, Bob McNab and Derek Dougan were known as opinionators not pundits. Over the past few years you cannot read anything about the BBC's coverage of football without reading a condemnation of how bland the punditry is and yet when Keane offers what we can only presume given his character is his honest opinion, he is attacked for supposed heresy against Manchester United and hypocrisy because he, too, was once sent off in debatable circumstances.

We know from his famous line about only dead fish going with the flow that he considers the contrarian stance is the honourable one. And while he may enjoy upsetting the cosiness of consensus there is no evidence that he was being insincere or spiteful. Indeed it is his sincerity and directness that makes him such a compelling television presence. If he really has "burned his bridges" with supporters who feel the load of one opinion they disagree with outweighs the substance of 12 years at Old Trafford, Wembley 1996, Anfield 1997, Turin 1999 and all that, well, frankly, good riddance.