Arsene Wenger saw Sir Alex Ferguson void coming - but can he fill it this season?

“It's a great void to fill for Manchester United, because the charisma and personality disappears suddenly in a club which has been dominated by it for such a long time. It's not an easy task to replace a person like that." – Arsene Wenger, May 2013.

Not for the first time, the Arsenal manager is left looking prescient. His tribute to the retiring Sir Alex Ferguson also took the form of analysis. Wenger understood the significance of his arch-enemy’s aura and the magnitude of his achievements. He did not explicitly say that David Moyes, the anointed one, lacked the managerial knight’s charisma and it would have taken still more foresight to predict Louis van Gaal’s appointment and conclude that he no longer possessed Ferguson’s judgment and galvanising qualities, but revisiting Wenger’s quotes from 2013 suggests he envisaged troubled times at Old Trafford.

Arsenal have been among the beneficiaries. In Ferguson’s last three years at Old Trafford, Manchester United took 47 more points than Arsenal. In the subsequent three, the Gunners have claimed 30 more. Victory at Old Trafford on Sunday would take the swing to 80 points. Arsenal have gone three games unbeaten against United, which may not sound much but is their best run in such clashes for nine years. Extend it to four by avoiding defeat and it will be their longest undefeated spell against United since Wenger’s finest team were on their way to becoming Invincibles.

That, in turn, shows why the role reversal is not complete yet and will not be until and unless Arsenal are crowned champions. Instead it is a tale of regression and relapsing at Old Trafford. Arsenal have advanced slightly. United have decayed dramatically.

To return to Wenger’s original assessment and the economist had a firm grasp of the figures. He was less farsighted when it came to the football. "What you can say still is that Manchester United are commercially and financially one of the strongest two or three clubs in the world, and that is still a good basis to start when you come in because you know the players are there, the team is there, and the potential is there," he said.

And indeed United’s turnover, projected to top £500 million this year, leaves them in a league with Real Madrid and Barcelona alone. It should be a platform for progress. Ferguson thought the squad he bequeathed prepared United for the future as well. He has railed against suggestions that Moyes inherited a mess. "You would have thought that I had left 11 corpses on the steps of a funeral," he wrote in his most recent book, Leading. "It's hilarious."

The calibre of player mattered less when he was still in situ. His presence ensured continuity and consistency at Old Trafford. Wenger has had the same effect at the Emirates. Contemporaries’ clubs emerged from ages of austerity, enforced because of the debts the Glazers incurred by buying United and the cost of building the Emirates respectively, at similar stages. Ferguson’s 2012 raid of Arsenal for Robin van Persie marked the first sign of United’s new expansiveness and acquisitiveness. It was the last of the near-annual departures of the Arsenal elite.

But, as Wenger alluded to, United have more leeway in the transfer market. They have spent £350 million since Ferguson retired and attempted to pay out rather more in their perpetual quest for Galacticos. Arsenal’s spending has been different: less scattergun, more strategic. If United have signed too many players, Wenger has bought too few. But, without wasting time pursuing Gareth Bale and Cristiano Ronaldo, he has at least recruited two second-tier superstars, in Mesut Ozil and Alexis Sanchez, and kept them. Under another regime, Ozil might have been the Arsenal Angel Di Maria, hastily dismissed as a mistake and offloaded at a loss. Instead, he is the most creative player in the country.

But a focus on the flagship players can obscure the long-term planning. It was long a forte of both, but in Ferguson’s final years, Wenger was the veteran quietly assembling the spine of a side. While Van Gaal could pick as few as three of Ferguson’s signings in Sunday’s starting 11, Wenger could name seven men bought before his rival hung up his hairdryer and an eighth, Hector Bellerin, who was in the youth system then.

Ferguson can cite David de Gea and Chris Smalling as evidence of a willingness to sign with his successors in mind but too many of his supposedly futuristic recruits in the 2009-13 years bear the look of optimistic punts. They mattered less then than now, after the old core of Van Persie, Paul Scholes, Ryan Giggs, Rio Ferdinand, Nemanja Vidic and Patrice Evra have gone. Rather than the basis for a champion team, Wilfried Zaha, Angelo Henriquez, Alexander Buttner, Shinji Kagawa, Nick Powell, Bebe and Mame Biram Diouf constituted a rather random assortment of misfits.

None of which rendered it inevitable that United’s decline would be as stark and as swift. The Ferguson factor now looks worth at least 15 points a season, if not over 20. If anyone guessed, perhaps it was the man who spent 17 years sparring with him, first getting the better of the Scot and then being floored by his punches.

Ferguson gave United a psychological edge and others an inferiority complex. He knocked Arsenal off their perch and serves as a warning to them about their future. “Who are they going to get to replace Arsene Wenger, who are they going to get who is better than Arsene Wenger?” he asked rhetorically in October. He spoke from experience. Over 26 years, he had illustrated the benefits of possessing an omnipotent manager who runs a club his way.

Because the crucial word in Wenger’s instant reaction to his rival’s departure was one of the shortest: void. Jose Mourinho seemed to fill it – at the Premier League summit rather than Old Trafford – before his own sudden fall from grace. A title race without either United or Chelsea offers an unprecedented opportunity to Arsenal. Yet if Leicester are crowned champions, the sense will be that Arsenal have stood still, forever third or fourth, while underdogs have accelerated past them and Ferguson’s former superpower reversed back into the pack.