Celtic: This was a reign bedeviled by ill fortune right from very start

But not until Andy Dorman slammed in a third goal for St Mirren five minutes from the end of Mowbray's nightmarish final encounter on Wednesday night was the need for immediate divorce crystalised.

The circumstances surrounding the 4-0 whipping laid bare the fact that Mowbray's methods weren't working, and weren't going to work. No Old Firm manager can survive his club's heaviest league defeat outside of derbies for three decades. Not when this comes against relegation-threatened, typically winless and goalless, opponents. Not when the result leaves them potentially 16 points behind their bitterest rivals. Not when the players Mowbray sent out appeared to chuck it, and seemed latterly to be chucked on willy-nilly as he fielded six attackers in a 3-1-6 formation. Not when he as good as conceded afterwards his philosophy of football, "trying to force the game and be expansive", wasn't the way to go in the Scottish league.

And not when the personnel Mowbray initially selected, and the 4-2-3-1 system he set it out in, had an element of experimentation at a juncture in the season when he simply had to see out wins to allow him to make it to the summer. He would see that experimentation as a courageous adherence to the bigger picture; others would describe it as making life unnecessarily difficult for himself. Indeed, therein lies one of the central issues. In taking responsibility for leaving his young central defence of 19-year-old Josh Thompson and 23-year-old Darren O'Dea exposed by his Light Brigade-style charge, Mowbray defended the "positive" attempts to retrieve a lost cause by saying you could be "a brave coach, or a negative coach".

For him, there are no inbetweens, even if you can also be a sensible coach, playing the percentage game, when the predicament demands. He would see this as an affront to his footballing sensibilities that amounted to a fixation with playing an attractive, attacking passing game. He was stubborn about how he felt he could deliver success and how he felt he should be able to conduct himself in the media; in each instance appealing to a higher set of principles – as a thoroughly good man does in all aspects of his life.

But in this grubby world of Scottish football, you need to get your hands dirty, need to indulge others in their games and not simply set your face against them. In press rooms as much as football fields up and down the country, Mowbray said what he thought and fielded teams he thought a collection of good players ... instead of giving primary consideration to how his thinking would play out. As a result, he was spun – one of his favourite words – and baited remorselessly in newsprint and had to watch as results regularly threatened to spin out of control.

That said, what most destroyed his hopes of achieving his footballing nirvana wasn't any flaw in his psyche, his professional approach, his demeanour, or whatever else will be claimed in the coming days. It was something altogether less tangible, and entirely uncontrollable: it was fate. No more and no less. There will be some who will home in on his transfer dealings, or outgoings, in pinpointing where it all went wrong. But his complete overhaul of the team was remedial work long overdue. Mowbray inherited a league- losing team from Gordon Strachan, his side winning only nine of the last 22 matches across 90 minutes – a run that included, incidentally, a cup defeat at St Mirren Park – and had the domestic third-string Co-operative Insurance Cup as their only booty. As a result, with the semi-finals of the Scottish Cup to come for a now Neil Lennon-led Celtic, there is still the possibility this season might prove more productive than the previous campaign to Mowbray's arrival in the only gauge that counts: silverware.

Of course, in terms of league points, it seems certain to be a whole heap worse, which fatally undermined the case for Mowbray continuing in his post. But it should be recognised the 46-year-old's downfall was his inability to arrest a decline long before set in motion, not be the architect of such. In some ways what damns him is that, in recruiting such as Robbie Keane, Morten Rasmussen, Landry Nguemo, Ki Seung Yung, Jos Hooiveld, Thomas Rogne and allowing Scott McDonald, Chris Killen, Paul Hartley, Barry Robson, Gary Caldwell and Stephen McManus to go elsewhere he appeared to attract better quality than he inherited. It damns him because he hasn't be able to gel his group into an effective unit.

That is where luck, or Mowbray's lack of it, comes in; the feeling it was simply pre-ordained it wouldn't happen for him as the club's 16th manager. He was never able to field what he would consider his strongest XI and, for the past two months of the season his three preferred central defenders – Hooiveld, Rogne, and Glenn Loovens – have hardly featured because of injury. The equivalent for Rangers would have been doing without David Weir, Madjid Bougherra and Danny Wilson at the same time for a quarter of the campaign.

Mowbray's teams have produced marginally better football than the sterile fare served up by the Celtic sides of last season, dominated opponents more often, and yet contrived to lose more league games, dribbling away points with alarming regularity. That has not always been their own doing, it should be noted. Celtic have suffered poor refereeing calls, most pointedly in the three derby games. Ultimately, though, even that simply strengthened the belief that Mowbray's management of Celtic was utterly bedeviled.

Supporters do not tolerate hard-luck stories. And after nine months, 13 defeats, nine draws and a modest 23 wins, Mowbray had no future with Celtic because the club's followers couldn't countenance one. They had given up on his vision they initially, cautiously, welcomed, and turned on him venomously at St Mirren Park. It was an unfortunate farewell for a man the Celtic faithful had always held in warm regard over the "honesty, integrity, humility and respect" he had shown in dealing with personal tragedy and immersing himself in all things Celtic when a player at the club in the early 1990s.

But humanity and decency don't sell season tickets. And the evaporation of the last vestiges of support he had made him a commercial liability.

Football boards don't continue to buy into long-term projects when they are headed by men who are box-office flops. By remaining in charge, Mowbray's very presence would have permeated a feel-bad factor just as season ticket renewal forms begin to flap through letterboxes. That wouldn't do for the Parkhead powerbrokers but, in doing the right thing by themselves, the best interests of Mowbray were also served.

It had become unpleasant to see the thrawn and agitated figure he had become. It wasn't him. Living apart from his England-based wife and three children – the youngest, at under a year, he has hardly spent any time with – was the sacrifice he was willing to make to guide a club whose "tremendous ... football history" he last night evoked in a statement. Despite the best intentions, Mowbray must carry the regret he could not add to that history.