Martin O'Neill had tried to project the sense that he was still in control. He was asked how he felt about Sunderland's chances of staying up and he looked his questioner in the eye and said, with a sudden and incongruous smile, that he was still "buoyant". Glass half-full or half-empty? "Three-quarters full," he said from the top table at Sunderland's press room. Then he thanked everyone and left the stage, oblivious to the fact they would be his final words as manager and the club were already planning his removal.

His words had carried a defiance that simply was not there as he stood on the touchline, as forlorn and inactive as maybe at any other time in his managerial career, watching his team go against pretty much everything upon which he has based his professional life. O'Neill's name used to be synonymous with teams that gave everything. He had the rare ability to energise an entire club, to instil confidence and belief. He had the enthusiasm and drive and sheer will that would coax lung-splitting efforts from his players. They loved to please him.

Yet this profession has plenty of examples of managers who have known nothing but success and then, almost inexplicably, encountered difficulties. O'Neill can be added to the list now. At Leicester, he took a middling second-tier club to one that had four successive top-10 finishes in the Premier League and reached three League Cup finals. At Celtic there were three Scottish titles and four cups, establishing him as their most successful manager since Jock Stein. O'Neill was shortlisted for the England job, frequently talked up as a possible replacement for Sir Alex Ferguson.

The difficult part is knowing what precisely has changed. One theory is that maybe the game has started to leave him behind and that his tactics have become outmoded. The immediate response to that is that he has previously demonstrated he possesses one of the sharpest minds in football.

Yet there is no point dressing it up as something that it is not: Sunderland, under his watch, had become a soft touch. Watching them meekly lose 1-0 to Manchester United in his final game, it was difficult not to wonder what had become of the O'Neill of yesteryear, the man once described as football's equivalent of the Duracell bunny because of the way he never stood still on the touchline. That was some manager back then, with some force of personality.

For Sunderland's owner, Ellis Short, perhaps the most alarming thing is that the team are deteriorating just at the point when the other relegation-threatened clubs – most notably Wigan Athletic, Southampton and Aston Villa – are improving to varying degrees.

O'Neill admitted on Saturdaythat his team's confidence was low. He tried to seize the positives from a slightly better second half but he was grasping at some very thin straws. The most relevant point he made was that a team struggling for self-belief can still press the ball, go into tackles, run hard. Sunderland had done none of these things and, though he did effect some form of change with his words at half-time, his own body language was alarming.

O'Neill used to be a young 60. He still is, in many respects, but the energy is no longer there, the ability to work wonders with a football team. Maybe he is missing his old mate John Robertson, in happier days the Peter Taylor to his Brian Clough. Or maybe it's just a fact of football life that someone who has excelled in the past can lose their way. Whatever it is, Short has clearly concluded that it is a full-blown slump rather than a blip.

O'Neill approached his first anniversary on a run of two wins from the last 21 league games going back to last season. At one point, Opta's statistics showed that only one other team, Fortuna Düsseldorf, had managed fewer shots on target throughout the top five leagues in Europe. Their current run, no wins in eight games, has seen them fall to 16th position, having played a game more than the clubs below them, Wigan and Villa, both a point behind.

Managers, like players, can peak early. It is 13 years since O'Neill's last win at Wembley with Leicester and 10 since he took Celtic to the Uefa Cup final. Sunderland should have been a snug fit – O'Neill, always contrary, had chosen them as his boyhood club simply to go against the grain – and initially the improvement was sharp and considerable. This season, however, has been a long and unhappy grind. Sunderland, to put it into context, have taken only six points all season after conceding the opening goal. There is a lack of competitive courage when that was once the quality that ran through O'Neill's teams.

O'Neill has been unable to coax the best from a £10m signing in Adam Johnson. Danny Rose, a loan player from Tottenham, probably has the most legitimate credentials to be their player of the year but the competition, bar possibly the goalkeeper Simon Mignolet, is sparse. James McClean has tailed off. It's the same for Stéphane Sessègnon. Danny Graham, signed from Swansea City in January, has yet to score.

Many will accuse Short of short-termism, of acting with too much haste, of falling into the trap of every impatient football club owner. For now, we cannot be sure whether there is a plan in place, with a replacement already identified. At the same time, this is not the O'Neill we used to revere. The old magic has all but disappeared this season.