David de Gea has been named in the PFA team of the year in five of the past six seasons. He has been Manchester United’s player of the year in four of the past five seasons. Last December, in Manchester United’s win at Arsenal, he equalled the Premier League record of 14 saves in a game. He is, by any measure, an exceptional goalkeeper, perhaps the best in the world. And yet De Gea’s position in the Spain starting lineup is far from certain after a run of indifferent form for the national side, the peak of which saw him allow a Cristiano Ronaldo shot to squirm under his body during Spain’s 3-3 draw with Portugal at the World Cup.

All goalkeepers make mistakes, of course, and the nature of the position means theirs tend to be remembered rather more distinctly than those of outfielders. But this was part of a pattern. De Gea does not play as well for Spain as he does for United. Trying to work out why that may be perhaps explains some of United’s difficulties under José Mourinho.

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There is one aspect of the game in which De Gea does not excel. His pass completion rate this season is 50%. Last season it was 57.5%. That is not terrible for a goalkeeper but it is not great. Ederson, for instance, was at 85.3% last season. Of course, Manchester City play a very different game to Manchester United. If Ederson went to United his score would drop and if De Gea went to City his score would rise. But even under Louis van Gaal, when there was an onus on playing out from the back, De Gea’s pass completion rate was only 60.7%. De Gea has brilliant reflexes and great positional sense but he is not especially comfortable with the ball at his feet.

That is not necessarily a problem, so long as the preferred style of the team and the goalkeeper are aligned. For an example of what can happen when they are not, you need only look at Petr Cech’s struggles at Arsenal this season.

Or take Cameroon in the late 80s, when they were blessed with arguably the two greatest goalkeepers in African history, Thomas N’Kono and Joseph-Antoine Bell. N’Kono liked to sit deep. He was a reactive goalkeeper who was noted, like De Gea, for his reflexes and his positional sense. Bell, by contrast, would regularly leave his box to sweep up behind the defensive line. Cameroon’s hierarchy could never quite decide between them, with the result that the defence would play with one style of goalkeeper in one game and a very different style in the next.

Bell blames Cameroon’s exit to England in the quarter-finals of the 1990 World Cup, when N’Kono played, on that deficiency. “If I’m not there and they move up then the through ball is really dangerous,” he explained. “You could see the way we were playing was not always quite the same. Players would go up if they knew they had somebody who could cover behind. But you have to have a coach who understands. It was not something we were always able to plan because sometimes you didn’t have a coach who could make a difference.” England’s equaliser and winner in that game came from penalties awarded after Gary Lineker had been fouled running on to through balls – exactly the sort of passes Bell’s style of proactive goalkeeping was designed to cut out.

Could that lie behind De Gea’s lack of international form? It is never wise to be too definitive about aspects of psychology but Spain play in a different way to United. They might not press quite like a Pep Guardiola team but they play with a much higher line than a Mourinho side. Even if De Gea is capable of playing like that, it requires an adjustment, and it seems reasonable to wonder whether that process unsettles him.

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Equally, Mourinho’s options at United are restricted. It would be a major change of policy for him to press high (although there were times at Porto when he did), and there has been no consistency of defensive selection at all over his two and a bit seasons in charge, but the presence of De Gea – and his excellence as a goalkeeper – means there is barely the option for even a slight tweak.

A defence sitting deep, in turn, means the midfield has to remain relatively deep if dangerous spaces are not to open up between the lines – which in part explains Paul Pogba’s frustrations. He has to be disciplined because there is no defence squeezing up – as there was, for instance, at Juventus, particularly under Antonio Conte – to fill the space behind him. And that deep-lying midfield in turn, explains why United are so often forced to play long and why, on a bad day, Romelu Lukaku can seem isolated.

The question is sometimes asked what would have happened if, in the summer of 2016, Guardiola had taken charge of United and Mourinho City. De Gea, it seems a fairly safe bet, would have been a Real Madrid player by now. A goalkeeper is never only a keeper of goals.