When Ole Gunnar Solskjaer's job as Manchester United manager was made permanent in March, almost everyone involved with the club was pleased. The Norwegian, a United legend as a player, had won his first six league games in charge and then remained unbeaten for six more. I won't embarrass certain pundits by name, but they explained that this brief run showed that Solskjaer had imbibed the club's spirit as a player at Alex Ferguson's knee, and that he was a worker and fighter who "understood" -- in a way that his predecessor Jose Mourinho apparently couldn't -- United's tradition of attacking football.

United's players also liked Solskjaer's relaxation of Mourinho's tight controls while Ed Woodward, the club's executive vice-chairman, was delighted to make the popular appointment, which got the phalanx of Solskjaer's former teammates-turned-pundits off his back. Yet United have faltered almost since the day the appointment was made. Nine games into the season, they stand just two points above the Premier League's relegation zone. But in fact, the appointment was wrongheaded even when Woodward made it.

It's a statistical truth that having been a good footballer does not make you a good manager. When Woodward gets around to choosing Solskjaer's successor, he'll need a different method. In fact, he should learn from last season's sensations in the Champions League, Ajax Amsterdam.

The classic managerial appointment in football is a white, male, former high-level player with a conservative haircut aged between 35 and 60. Clubs know that if they choose someone with that profile, they won't be blamed too much even if the appointment turns out to be terrible because at least they will have failed in the traditional way. Yet there never has been any evidence to support the appointment of former high-level players.

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Way back in 1995, Stefan Szymanski, my co-author on Soccernomics, carried out a study of 209 managers in English football from 1974 to 1994, looking at which ones consistently finished higher in the league than their teams' wage bills predicted. "I looked at each manager''s football career, first as a player (including number of games played, goals scored, position on the field, international appearances, number of clubs played for) and then as a manager (years of experience, number of clubs played for, and age while in management)," Szymanski said. "Playing history provides almost no guide except that defenders and goalkeepers in particular do not do well: most managers were midfielders, while forwards are slightly more successful than the average."

Some former star players, like Kenny Dalglish and nowadays, Pep Guardiola and Zinedine Zidane, did well as managers. Others, like Bobby Moore (and more recently, Diego Maradona and Thierry Henry), have done badly. Taken overall, a good career as a player predicts neither success nor failure as a manager. The two jobs just don't seem to have much to do with each other. As Arrigo Sacchi, a terrible player turned great manager of Milan, phrased it, "You don't need to have been a horse to be a jockey."

A horse's knowledge doesn't help a jockey, either. Here is one player-turned-manager testifying anonymously in Sue Bridgewater's book Football Management: "I got the job and on the first day I showed up and the secretary let me into my office, the manager's office, with a phone in [it] and I didn't know where I was supposed to start. I knew about football, I could do the on-pitch things, but I had never worked in an office. I just sat there and waited for something to happen, but no one came in so after a while I picked up the phone and rang my Mum."

Even this man's claim that "I knew about football, I could do the on-pitch things" is dubious. Does Maradona know more about football than Mourinho? Did Roy Keane's knack for geeing up teammates on the field translate once he had become a jockey?

Mourinho remains, on a match for match basis, among the most successful coaches in football history. When Milan's then-coach Carlo Ancelotti noted his almost nonexistent record as a player, the Portuguese replied, "I don't see the connection. My dentist is the best in the world, and yet he's never had a particularly bad toothache."

Asked why failed players often become good coaches, Mourinho said, "more time to study." They also have to have provided some evidence that they are good coaches, because nobody is going to hire them based on their playing careers.

Even Ancelotti, who was a canny midfielder for Sacchi in Milan, seems to have changed his mind about the usefulness of having played. He told me in 2013, when he was coaching Real Madrid, that "experience as a player can help you just in one situation. I can understand what the players are thinking, but the job is different. You have to study to be a manager."

In Germany, former players have lost their monopoly on managerial jobs. On the German football federation's annual training course to certify professional coaches, an average of 16 of the 24 places are now reserved for people who didn't play professionally. The head of the course, Frank Wormuth, told Dutch website DeCorrespondent.nl that although it helps to know "the smell of the stables" in professional football, "that's only one aspect of being a coach. How are you pedagogically, analytically, communicatively? Ex-pros often have less of an eye for that."

Successful German coaches in 2019 include the national team manager Joachim Löw (who played 52 Bundesliga games), Thomas Tuchel of Paris Saint-Germain (eight games in the 2. Bundesliga) and Julian Nagelsmann, 32-year-old coach of RB Leipzig, who never played a professional match. Nagelsmann's career would have been unthinkable in most major football countries. It's their loss.

Solskjaer (who before United had a disastrous eight-month stint managing Cardiff) now follows Bryan Robson, Roy Keane, Gary Neville, Paul Scholes, Steve Bruce and Jaap Stam (currently in midtable of the Dutch league with Feyenoord) in having failed to absorb Ferguson's managerial skills from his time as a player under the Man United legend. It's almost as if there's a pattern here. Even Solskjaer's much-praised willingness to give his players more on-field freedom than Mourinho had wasn't necessarily a good sign. Mourinho is a high-control manager; Solskjaer a laxer one. The former method tends to improve organisation, whereas the latter allows more creativity. Each has its pros and cons.

One club that has finally figured out how to choose a manager is Ajax. The Amsterdammers used to cling to the notion that only former Ajax players could possibly know how to run the club. After years of failure, the notion was junked in 2016. The outsider coach Peter Bosz promptly put together a thrilling modern team that reached the Europa League final in 2017 but when Bosz left, Ajax replaced him with their former player Marcel Keizer, untested as a top-level coach. Only after Keizer's rapid failure did Ajax appoint another outsider, Erik ten Hag, and his appointment attracted none of the plaudits that Solskjaer received.

The uncharismatic, bald, former journeyman full-back spoke in a "country bumpkin" eastern accent, uttering almost impenetrable jargon about "half-spaces" and "the rest-defence." Yet he put together a brilliant team that reached the semifinals of the Champions League. Clearly, he benefited from the fortunes Ajax had spent signing players like Dusan Tadic, Hakim Ziyech and Daley Blind -- the quality of players almost always matters more than the quality of the manager -- but Ten Hag got his squad playing a hypermodern, well-thought-out system.

By "half-spaces," he meant the idea that a field doesn't simply divide into the usual three zones (left, centre, right) but into five. Like Guardiola, he thinks endlessly about which player in which line should occupy which zone, and when. "The rest-defence" refers to the number of players who have to be covering the three central defensive zones from the moment the team loses the ball. Ajax always have three, which is why this extreme attacking team is so rarely surprised on the counter. And Ten Hag has taught his defenders the difficult art of "defending forward" -- advancing towards the opponent with the ball and tackling him fast, so as to set up new attacks.

In the long term, clubs brave enough to appoint the Ten Hags and Nagelsmanns will gain an edge over the clubs appointing the Solskjaers and Keanes. But in the short term, appointing the popular ex-player will always be the easier choice.