THERE will be few expressions of surprise at today's formal announcement that Ronny Deila has 'decided' to leave his post as manager of Celtic in the summer. The only surprise may be that it was his decision to make.

Any other outcome was rendered inconceivable in the wake of Sunday's Old Firm William Hill Scottish Cup semi-final defeat to Rangers, but in truth the seeds of the Norwegian's demise were sown far, far earlier. In football terms, Deila has been a 'dead man walking' ever since he oversaw the club's second successive non-qualification for the Champions League group stages, and presided over an underwhelming Europa League campaign which saw the club relegated to second best against his former Norwegian Tippeligaen rivals Molde.

With an eight-point lead at the top of the table, Deila will be permitted the chance to clinch the Ladbrokes Premiership title. This they will surely achieve, enough for the 40-year-old to walk off back to the land of the midnight sun having fulfilled his base job description, of winning two back-to-back Scottish titles, taking Celtic onto five in a row. In that first season, he managed a League and Cup double. But the Parkhead club expected so much more than that and Deila has ultimately failed to deliver.

Speculation on his successor has been under way for months already, but first it seems only fair to offer an appreciation of Deila's time at the club. Arriving in the summer of 2014 with a rigid tactical plan and grand ideas about nutrition, training and diet which paid dividends at Stromsgodset, he struggled to impose all those ideas at such a massive club. At times, particularly during that first season, Deila was a hugely popular figure, his trademark after-match roar capturing the fans' imagination. But his time has ended with a whimper: for a figure with such strident first principles, instead the club in recent times has appeared strangely directionless, while a recruitment arm which once functioned so smoothly when unearthing the likes of Victor Wanyama and Virgil van Dijk left him with a lop-sided job lot of talented young players which he has struggled to shoehorn into an effective team.

While the personal development of players such as Leigh Griffiths and Kieran Tierney have been plusses, the Old Firm match with Rangers on Sunday summed up the club's problems, with the Premiership champions elect seemingly content to surrender the football to a championship side, relying on counter attacks and moments of genius to win the match.

It is often said that you always know that a manager's time is up, because it happens just after the fans started concertedly calling for the head of the chief executive and the board. And that certainly applies in this case. Responsibility for Deila's appointment - he was originally interviewed for the assistant manager's job - falls squarely at the door of Peter Lawwell and the club's board and they were left with little option to cut their losses. While I am never one to call for people to lose their jobs, they must also ask themselves if this season would have panned out differently if they had acted sooner.

In any case, the twin imperative to put bums back on seats, and renew the club's focus ahead of the return of Rangers to the Scottish top flight, also necessitates the right choice for his successor and in this they have plenty to ponder. Would the return of Neil Lennon, after a chastening time at Bolton Wanderers, be seen as the popular return of the prodigal son or merely a step back the way? Do the club have the clout and the ambition to land David Moyes ahead of a sizeable queue south of the border? Is the end of his 2016 involvement the time for Roy Keane, a known Dermot Desmond ally, to take the throne? If they are ruled out, what about Malky Mackay, Owen Coyle or Alan Stubbs? Has the route of bringing in unknown foreign outsiders with impressive track records fallen by the wayside with Deila?

Whatever happens, the starter's pistol has been fired in the hunt to find Celtic's next manager and the board know there can be no mis-steps this time. Assuming there are no catastrophes in the league, they have the 12 weeks until their first Champions League qualifier to find him.