This weekend, in keeping with most others, a much-scrutinised if seldom enlightening ritual will have been enacted once more: the post-match managerial interview. It’s a strange phenomenon, if you think about it, but coverage of a football match without quizzing the manager at the end would now seem as unnatural as the police not questioning the prime suspect in a crime.

It has become an integral part of the mechanism that bestows upon managers a position somewhere between a god and a fool, depending on the result. But once a game has started, what can a manager actually do? Scream from the sidelines at players who are too busy playing to take any notice. Give a half-time team talk that takes one of two forms: keep doing what you’re doing or pull yourselves together. And make up to three substitutions – almost always after 60 minutes of play.

It is almost a Canutian endeavour: trying to stop the tide by shouting at it. But nonetheless we have a deep need to establish accountability in random situations, and that’s the manager’s job – to take responsibility. And they’re well paid for it, even if their stay in the post is, on average, little more than two years.

Of course, the flow of a football game is not entirely random, and it is down to the manager to institute certain formations, styles and patterns of play. But again we tend to overstate their impact, which is what they encourage us to do – unless they are on a losing streak. For it is in the manager’s interests to be seen as the omnipotent architect of success.

There are rare examples of managers whose careers tend to support that picture, most notably Sir Alex Ferguson, though of course he was said to be a match away from the sack at Manchester United before he embarked on his glory years. But a more instructive case is José Mourinho.

By any standards, Mourinho is a supremely successful manager, with a list of major trophies won at Porto, Chelsea, Internazionale, Real Madrid and Manchester United. Perhaps his greatest success, though, has been in convincing everyone – his bosses, the media and not least his players – that he is able to take control of a club and drive it to the top. He famously announced that he was “special” when he arrived in the Premier League, and winning the title in his first year seemed to confirm that he was.

Another way of looking at it would be to say that Mourinho took over a club that was on the verge of major success, and he was able to recruit a number of major players thanks to the largesse of the club’s billionaire owner, Roman Abramovich. Verge is not the same as pinnacle, though, and let’s say Mourinho made the difference.

Then three years after his arrival he was gone, having fallen out with the owner. There is no doubt that he has continued to deliver trophies at other clubs but with each new triumph, followed by player discontent and his departure, Mourinho has grown steadily more mortal. Now at United, after spending £360m (£285m net), he looks much like any other top-flight manager – desperate to make his extravagantly paid squad of underperforming stars gel into something more than the sum of their parts.

For more than a decade he had a psychological edge because the sheer magical strength of his image seemed to cast a debilitating spell on his opponents. But now that advantage is gone, what is left? Tactics, fitness, diet, preparation, and all the things that every other manager and coaching staff are working on too.

When a new manager arrives, such as Unai Emery at Arsenal, it is customary to note how much fitter the players suddenly look. Yet we can be sure that under Arsène Wenger, a man who ate and slept performance statistics, the players were not lolling around on sun loungers. The strange thing is that if Emery leads his team to a Champions League qualifying spot – the achievement Wenger secured for years on end while being said to lack ambition – he will be deemed a success.

But the finest illustration of the mysterious and perhaps illusory role of managers is the man Mourinho originally replaced at Chelsea, Claudio Ranieri. When their paths later crossed in Italy, Mourinho would dismiss him as an old man “who hasn’t won anything”. Then, after a much-travelled career, in which he seemed to specialise in coming second or third, he arrived in 2015 at Leicester, a team that had only just survived relegation the previous season. As Ranieri was known as the Tinkerman for his habit of constantly changing team lineups when he was at Chelsea, it was a safe bet that he would change things around.

But no major signings came in, Ranieri kept a settled team, the same one that was built by his predecessor, Nigel Pearson, and nor did he make any discernible changes in tactics or training. As we know, Leicester went on to win the Premier League, in what was arguably the greatest upset and most extraordinary accomplishment in modern-day football.

Did that make Ranieri, who had never quite made it to the top in his previous 15 managerial jobs, a genius strategist? If so, it did not stop him from being sacked the following season. It’s worth bearing that in mind the next time a manager is hailed as inspired for bringing on a substitute who scores, or damned because a left-back mistimes a tackle in the 90th minute. Managers can have an effect over and above the players at their disposal but it’s usually less than we think and it is, almost without exception, as fleeting as Ranieri’s tenure.