Under Kevin one thing is for sure: nothing is certain

The hair has greyed, but he still possesses the youthful exuberance of a footballing Harry Potter. Certainly there are more than enough believers in his wizardry on Tyneside. Yet if he has a failing it is that his enthusiasm tends to wane. He falls in and out of love with the object of his affections like a teenager. As another former England manager, Graham Taylor, put it wryly: "If, after a certain amount of time, he feels things are not moving in the right direction, there will be no need to sack him."

While there is unlikely to be another handsome compensation pay-out – like that paid to Allardyce – should success fall short of anticipation, owner Mike Ashley will know that wherever Keegan goes, unpredictability is his travelling partner. His departures from Newcastle once before, in 1997, Manchester City, in 2005, and England, in 2002, were all unexpected, at least to the fans. The only club he left under what can be considered "normal" circumstances was Fulham, when he was offered the England job. Hence the barbs have always attached themselves to him: Keegan can't hack it when things get tough, they claim.

So, what is the truth? At Friday's press conference Keegan insisted: "I left [Newcastle] because of circumstances last time... because I didn't think the club was going where I wanted it to." Yet, as so often with Keegan, nothing is straightforward. In his autobiography, he starts off by stating that "there were no threats from me. I didn't jump. I was pushed [by the board]." Yet he then goes on to recall that "I had offered to step aside three or four times in the five years I'd been at Newcastle". One such occasion followed 1995-96, the season in which his team capitulated to Manchester United after holding a 12-point lead in the title race. "I said not only was I quite happy to resign, but I would welcome it." The board may, indeed, have "pushed" him eventually but as Keegan admits himself, it was at the time of the club's flotation, and those concerned understandably desired stability. That is a concept which, long-term, is not something you would associate with Keegan.

In one sense, you could say that he is both honest to himself and to his employers. One can condemn the timing of his walk-out from England in 2000, just after the defeat by Germany, and four days before the next international, but not his rationale. As he emphasised at the time, he did the job "to the best of my ability... I am not the man to take it that stage further". As he reiterated on Friday, "with England, I wasn't enjoying it".

The following year, re-energised, he was installed at City who achieved promotion under him, just as Newcastle and Fulham had during his stewardship. He had always said he planned to retire from management when his contract there expired at the end of the next season. He just went early "by mutual agreement", with City in 12th place. On Friday, he elaborated on that, claiming that after achieving promotion with City, he stabilised the club. "The reason I left was I wouldn't sign a five-year contract. I wasn't sacked. I left."

The irony was that, in the knowledge that Keegan would be departing sooner or later, City had earlier identified his likely successor: one Sam Allardyce. It was not to be. Stuart Pearce was given the chance. He was followed by Sven Goran Eriksson, who would succeed Keegan with England. Such are the mysteries of football management; ones that are all the more complex when Keegan is involved.

If you analyse his career, he appears to relish the initial challenge but is not a man who has ever appeared content to engage cruise control. In 1992, chairman Sir John Hall explained to him that Newcastle "would fold if it fell through the trapdoor" into the then Third Division. "There are only two people who can save Newcastle United Football Club, and we are talking on the telephone." Keegan could never have imagined he would not only become a saviour but leave an extensive legacy; then watch it squandered by board members and successive managers.

Ashley has asked him to start the process again, albeit from a less drastic position. Succeed or fail, the club's owner is assured of one thing: his manager is unlikely to hang around, either to accept the plaudits or rebuff the seemingly unthinkable now, the Toon Army's chorus of reproof.