Sir Alex Ferguson needs to figure out that two into one won't go

The Outsider, by Jonathan Wilson, is a forensic study of the position, the people who have played there and the intricacies of the role. The chapter about Thomas Nkono and Joseph-Antoine Bell, "Tommy and JoJo", may be of particular interest to Ferguson at a time when the Manchester United manager appears to be suffering from an unusual lack of clarity.

Nkono and Bell were rivals in the Cameroon team for almost two decades, and are probably the best African goalkeepers there have ever been. In different circumstances they would have been undisputed number ones in their own right. It was just their misfortune to come up against one another at the same time. Just as David de Gea and Anders Lindegaard are doing at United, they shared the role. Both men came to think it did nothing but confuse the team's defence.

Peter Shilton and Ray Clemence used to think the same when Ron Greenwood was alternating them. On one occasion – England's 4-3 defeat in Austria in 1979 – Greenwood selected Shilton for the first half and Clemence for the second. Yet the policy did not take into account that a defence needs an understanding with its goalkeeper, and vice versa, and that for this position – perhaps more than any other on the pitch – there is clear sense in nominating a regular first-choice and sticking with him. The bottom line is, goalkeeper rotation does not work. It probably speaks for itself that there are so few other examples (César Sánchez and the young Iker Casillas is one, though Real Madrid's coach, Vicente del Bosque, soon realised "San Iker" was the better option) at a time when Ferguson continues to swap around De Gea and Lindegaard while complaining he can barely remember United being so vulnerable defensively.

There are not many times when Ferguson can be accused of sustained indecision, but this is one of them and the longer it goes on the more confusing it gets.

De Gea played the first two games of the season but was dropped because of a mistake against Fulham. Lindegaard came in for the next two and then they took turns, one after the other, for the following four games. De Gea was back for the next five, then Lindegaard for one, and then De Gea played three in a row before needing a wisdom tooth removing.

Lindegaard kept his place for five games until De Gea was recalled for the match against Cluj on Wednesday. United, meanwhile, have conceded 33 goals in the various competitions and been derided by their manager for Cartoon Cavalcade defending. It is not an exact science, but it is difficult not to think the two things are linked.

The lesson of history is fairly clear. Of Ferguson's 12 titles, Peter Schmeichel played every game in 1992?93, missed two in 1993-94, 1995-96 and 1996?97, and four in 1998-99. Post?Schmeichel, United struggled to fill the position, but Mark Bosnich still played 23 of the 38 league fixtures in 1999-2000, the year of Massimo Taibi. Fabien Barthez played 32 times the next season and 30 in 2002-03. Edwin van der Sar made 32, 29, 33 and 33 appearances in his title-winning seasons.

This is the pattern of just about every successful team. Petr Cech played in all but 11 games during three championships with Chelsea. Joe Hart did not miss a single match for Manchester City last season.

How Ferguson must wish he had paid the ￡100,000 that Shrewsbury Town wanted for Hart a few years back. "We all make mistakes," he says.

That is not to say Hart has been flawless this season. In fact, broadening the argument, it has been a pretty undistinguished campaign so far for goalkeeping in the Premier League. Rob Green, Pepe Reina, Shay Given and Adam Federici have all, at one point of another, lost their places because of mistakes. If Southampton are trying to work out why they have been in the relegation zone, the performances of Kelvin Davis, Artur Boruc and Paulo Gazzaniga represent Exhibit A. For Arsenal, Wojciech Szczesny and Vito Mannone have taken turns making supporters pine for the security of David Seaman. Tim Howard, Jussi Jaaskelainen, Mark Schwarzer, Simon Mignolet and Ali al-Habsi have all let in soft goals. It is a long list.

Yet Ferguson is currently the only manager unable to decide on a first-choice and it is rare to see him this indecisive. Just as perplexing, there is little consistency. At the start of last season, when De Gea genuinely looked a danger to his own team, Ferguson persisted with him until a particularly bad run at Christmas. This season, one mistake and the Spaniard was out.

The problem for Ferguson is that neither De Gea nor Lindegaard have an exceptional case. A few days ago I asked a former pro – a striker who studied goalkeepers for potential weaknesses – who he would choose. "Neither," he replied. "They're not good enough for Manchester United."

Gary Neville seems unenthused, too, noting the goals his old club have conceded from set pieces and the absence of a goalkeeper dominating the penalty area. Neville knows what he is talking about. "The defenders will be nervous about what's behind them," he says.

So who should it be? At a push, I'd say De Gea would edge the kind of vote London's now defunct Evening News held in 1978, when it asked 22 players whom they would choose for England: Shilton or Clemence? Nine voted for Shilton and two for Clemence, one being his Liverpool team-mate David Johnson. Eleven could not decide.

De Gea has, after all, shown flashes of brilliance and, though far from perfect, who would expect him to be, just a month after he turned 22? His potential is considerable and it would be a poor judge who could not see it. It is just that it is the manager after Ferguson – or maybe even further down the line – who will probably see it realised on a more consistent basis. For now, De Gea has been lucky United have had enough firepower over the past couple of seasons to ensure the goals conceded from his mistakes have not cost the team more points.

Equally, however, it was always going to be the case that a new player, in a new country, would make errors. Overall, he has done about as well as could be expected since that bruising crash-course two autumns ago. It is easy, in fact, to feel a little sorry for him. It cannot be helping his confidence that his manager keeps playing football's equivalent of musical chairs.

More than anything, it disrupts the team. Goalkeepers are different. This is the point Bell and Nkono make. Nkono, for example, always wanted a man on each post at set pieces, whereas Bell left them free. Bell liked to sweep up behind his defence, allowing them to play a higher line. With Nkono, the back four had to drop deeper.

It was the same for Shilton and Clemence. Shilton hung back, Clemence moved forward. Shilton always wanted the area in front of him to be free, whereas Clemence liked the defenders nearer to him. Subtle yet important differences.

De Gea, talented yet erratic, and Lindegaard, steady but not a man for the outstanding save, have their own styles. Together, they have been part of a team that are top of the league. Ultimately, though, the policy isn't working. Just look at United's goals-against column, 10 worse than City's.

What is most surprising is that a manager of Ferguson's normally clear judgment is involved. He says it is because he has confidence in both goalkeepers. The impression it actually leaves is that he does not have total confidence in either.