Martin O'Neill has always been a man who put principle before profit so it is probably safe to assume that he did not walk out on Aston Villa five days before the start of the season because he was not offered a pay rise.

When it comes to fallouts with chairmen over transfer policy the Northern Irishman has form having left Norwich City after six months following just such a row in 1995.

Everything points to Monday's departure being based on frustration at Villa's intention to sell James Milner to Manchester City -- for a healthy profit -- and possibly to listen to offers for Ashley Young.

So while the timing was a shock, his decision to go should not have been as he has long warned that if Villa were to hold serious hopes of breaking into the Premier League's top four then the club should be buying, not selling.

Famously loyal, infectiously enthusiastic, fiercely proud, a great motivator and an honest and entertaining pundit, he is also seen by many as the natural successor to Alex Ferguson as Manchester United manager.

O'Neill has been something of a media and general-public darling ever since he rejected tempting offers to stay with minor-league Wycombe Wanderers while he cut his managerial teeth following a trophy-laden playing career.

Like the man he served under during his heyday as a player, Brian Clough, O'Neill is unquestionably gifted at bringing the best out of previously unremarkable players and building success with teams that other managers have failed with.

Wycombe were minor league makeweights when O'Neill took over in 1990 but by the time he left five years later they had won the FA Trophy twice and were established in the Football League.

O'Neill had seen first-hand how far team spirit and determination could carry a side, not only at Forest but with Northern Ireland, who he captained during their glorious 1982 World Cup campaign.

He repaid the faith shown in him by his players by rejecting approaches from bigger clubs to see Wycombe safely into the third division.

He finally made the step up to Norwich, one of his former clubs as a player, but it proved a short-lived association as he walked out in frustration after six months.

Five happy years at Leicester City followed, when two League Cup triumphs, promotion and then consistent mid-table finishes in the Premier League had the fans successfully campaigning for him to stay when a move to Leeds United looked on the cards after he had already said no to the idea of managing Northern Ireland on a part-time basis.

O'Neill then took over at Celtic where a welter of Scottish silverware was backed up by a run to the UEFA Cup final. After being regularly linked with just about every Premier League vacancy, as well as the England job, he stepped out of the game for a year in 2005 to look after his wife, who had cancer.

He returned with Villa at the start of the 2006/07 season and after a sticky first year led them to three successive top-six finishes, with last season also featuring runs to the final of the League Cup and semi-finals of the FA Cup.

Being so close, so often, without making the final step into the big four, left him -- often literally -- hopping mad despite owner Randy Lerner pumping millions into the club.

He made no secret of his frustration at having to sell Gareth Barry last year while the likely departure of Milner to Manchester City along with talk of a possible exit of Young had stirred the Internet forums into predictions of his possible resignation as long ago as March.

Many Villa fans were less than distraught at the prospect of losing a man who had spent huge sums, almost entirely on British players and most of them already established Premier League performers, with very little home-developed talent given a shot at the first team.

His touchline histrionics, while entertaining, were considered by some as unhelpful in conveying calm instruction to his players, while Villa's limited tactical options have long been a point of frustration.

Despite those shortcomings, however, just about any British club with a vacancy this season is likely to pencil in O'Neill at the top of their short list of candidates.