Manchester United have just agreed to take part in a summer tournament with Real Madrid, Liverpool, Internazionale and Manchester City in the United States, and could be forgiven for wondering if one of their possible opponents has just had a dig at their expense. "The Blues join an A-list cast of European footballing superpowers," was how City's website trumpeted news of their own involvement. All the next line needed was a winking emoticon for extra effect. "Neighbours Manchester United will also be appearing," it said.

It has been that kind of season at Old Trafford (City, in fairness, removed the key sentence after the Manchester Evening News got to hear about it) and the announcement from Nemanja Vidic that this will be his final season has sent the curtains trembling again.

Vidic has been a centre-half of authentic greatness, often the best pure defender in English football, and even if his most outstanding qualities are now talked about in the past sense, his standards were so high in the first place it has been a surprise to see how many people have taken a diminishing of his powers to mean he no longer belongs at this level. He is still a highly capable performer and there is a portfolio of evidence that United are considerably better with him in the team than without him.

Martin Keown has accused him of "running away" and, though that kind of terminology misjudges everything we know about Vidic, the former Arsenal player makes some relevant points. "Vidic is not old, he is 32, so he should be rolling up his sleeves and fighting for the club that allowed him to win five Premier League titles, the Champions League, three League Cups and one Club World Cup. When I was at Arsenal, I clung on with my fingernails, staying until I really had nothing left to offer at 37 years old. So why is Vidic running away? He has been a fantastic player for them – one of the best defenders in Premier League history – but he is turning his back on them."

What is certainly true is that it never reflects well on any elite club when the captain decides he wants "a new challenge". Vidic might not be quite the player he was, in an age when Prozone data indicates the sport is 20% faster than his first couple of seasons in Manchester, but it is a mistake to think the Serb could not still make a valuable contribution. He is in far better nick than, say, Rio Ferdinand three years ago, or John Terry last season, and however much United try to fluff it up as a mutual decision it is plainly Vidic's doing, at a time when the team are enduring a slump, not a blip, and nobody at Old Trafford can be sure how long it will be before a sense of order is restored. He deserves better than to be accused of treachery, but it does smack of someone who suspects one bad season could turn into a longer period of drift.

United reached the Champions League final three times with Vidic at the heart of their defence. They are now seventh in their domestic league, 19 points worse off than this time last year, and skidding towards the potential horrors of starting next season's Europa League in the qualifying round that included Differdange 03 of Luxembourg, FK Kukësi of Albania and Aktobe, the Kazakh champions, at the corresponding stage last summer.

If anything, it would be more of a surprise if a few of Vidic's team-mates – and not just Wayne Rooney – had also contemplated "running away". It is tempting, just for starters, to wonder what a player of Robin van Persie's ambitions has made of the past eight months.

The next announcement will surely be from Ferdinand, whose retirement will strike another line through what was once the most formidable central defensive partnership in the land. Patrice Evra is approaching 33 and another player in gradual decline, albeit not so obviously as Ferdinand. The flecks of silver in Ryan Giggs's hair are expanding and suddenly it is not easy to look through United's squad and find a vast amount of natural leadership.

Vidic's role at Old Trafford extends beyond being simply the guy who gives everything on the pitch and reminds the sport that the perfectly timed tackle can still be a thing of beauty. He is the player who sets the standards at Old Trafford and lifts others with his force of personality. Not the only hard-boiled egg in his sport, but definitely in for a full 10 minutes.

He is also one of the last of his type, once diving full-length in a match against Swansea in his desperation to block a point-blank shot with his face. Kenny Burns, Nottingham Forest's scarecrow of a centre-half back in the day, once told me a story about psyching out Kevin Keegan before the 1980 European Cup final by taking out his false teeth in the tunnel and smiling at his opponent. Vidic, I always fancied, would have enjoyed that story.

His presence is considerable and nobody can just assume Phil Jones, Chris Smalling and Jonny Evans will fill the void seamlessly. As talented as they are, it is hard to make any argument they are at the level United have had before. Not yet, anyway. Each has a habit of giving the opposition something, plus a tendency to pick up injuries.

What is clear – and this is becoming a recurring theme at Old Trafford – is that the next period of change is going to have to be substantial. Ed Woodward, United's chief executive, has been on a good run with the £37m deal for Juan Mata, persuading Adnan Januzaj to repel the advances of wealthier clubs and not blinking once in the staring contest with Wayne Rooney's advisers. Yet the awkward truth for David Moyes is that the team he inherited at the end of last season – the champions by 11 points, lest it be forgotten – would be as close in points to the relegation places as top position had City enjoyed a more profitable week.

A frontline quartet of Rooney, Mata, Januzaj and Van Persie could trouble most defences but the neglect of United's midfield pre-dates Januzaj's teens and it is not going to be straightforward, or cheap, replacing Vidic satisfactorily. Eliaquim Mangala of Porto has been in United's thoughts, but that would mean getting in ahead of City. Luke Shaw has been identified as the best possible replacement for Evra now that Leighton Baines is off-limits, but again it is not straightforward. If Shaw prefers Chelsea, as United suspect, it is going to be hard to break that spell if one club is in the Champions League and the other is fixing a smile and trying to create the impression that this period of turmoil has not become their normal state.

For Moyes, it is about making it manageable turmoil before a period of recovery can begin, and that process requires strong leadership on the pitch and off it. Perhaps it will become clearer how important Vidic is when he is no longer around.

WHo is the real laughing stock at Newcastle United?

Not for the first time, it is tempting to wonder whether the statue of Sir Bobby Robson outside St James' Park should have his hands covering his eyes, rather than negotiating his trouser pockets, bearing in mind some of the birdbrained goings-on behind the stadium walls.

The most startling point about the Joe Kinnear tragicomedy was that it was so utterly avoidable, both times, if only Mike Ashley had carried out the kind of background checks that might ordinarily be expected for a man who has made millions through business.

Maybe he could have put in a call to Nottingham Forest, for example, and he might have learned that Kinnear's time at the City Ground was just as chaotic and half-baked as everything we have subsequently seen at Newcastle. It just escaped the same level of attention because it occurred towards the bottom end of the Championship, away from the glare of the Premier League.

Kinnear set the tone on his first day at Forest when he became hopelessly lost driving around the city while the club's directors waited for him to pitch up to sign the relevant paperwork. More than once, he rang ahead for directions without picking up the trail. The place was covered in darkness by the time the club had a lightbulb moment to help him out. Or a thousand lightbulb moments. The chief executive ended up switching on the City Ground's floodlights to guide him in.

Kinnear promised "sexy signings", mentioning clubs and targets who would later seem totally nonplussed (the sexiest it got was Andy Impey on a free transfer from West Ham's reserves). The fans who questioned him were called "morons" and the correspondent on the Nottingham Evening Post experienced Kinnear's full-on rage long before the words "which one's Simon Bird?" became part of the modern-day Newcastle story.

They also remember one defeat at Forest when Kinnear worked himself into such a froth he picked out one player, Chris Doig, for some particularly harsh criticism. Doig had been one of the unused substitutes. As for Yohan Kebab, Derek Lambeeze, Ammomobi, Ben Afra et al at Newcastle, they are merely extensions of Jean-Louis Paul (Matthieu Louis-Jean) and Marvin Harwood (Marlon Harewood). Kinnear left Forest amid escalating protests – any of this sound familiar? – and the kind of thud and blunder that would put off any football club owner operating with common sense.

Now we hear stories of Kinnear leaving a missed call on Alan Pardew's mobile phone one night last August, as if unaware Newcastle were actually playing a Capital One Cup tie in Morecambe at the time. There was the account of him watching a game at Birmingham and asking Lee Clark about Shane Ferguson only to be told that, well, all a bit awkward, but the player was actually on loan from Newcastle in the first place.

Yet the daddy of them all will always be that little masterpiece from his first spell at St James' when he elaborated on his time as Nepal's national coach and the shocking events when the king's son – "one of my closest friends," Kinnear recalled – was instructed to "marry some bird … he wanted to marry someone else but he couldn't … like the usual crap. So what did he do? He killed them all and then blew his own brains out. I got out of there about three days after and I have never been back."

Journalistic gold, bar one inconvenient detail. Kinnear managed Nepal in 1987. Prince Dipendra killed himself and eight members of his family in June 2001, when Kinnear was managing Luton.

He reminds me of the television guy talking into the camera while someone stands in the background with a placard reading: "Don't believe a word." But if Kinnear is to be remembered as one of football's great absurdities, what does that say for the man who employed him twice?