The question, often, is not the what or the how but the why. On Sunday, after Manchester United had staggered to a 1-1 draw against Sunderland and Queens Park Rangers had been hammered 4-0 by Tottenham, there was much carping about 3-5-2, the formation United had used in taking one point from their first two Premier League games of the season, and QPR in losing both their opening matches.

That Hull City had played 3-5-2 in beating QPR and that Steve Bruce’s side reached the FA Cup final last season playing with this formation tended to be overlooked amid the carping. It is rarely the system to blame, but the personnel and their application of it.

The biggest question about Louis van Gaal’s immediate implementation of 3-5-2 is the why. It was a formation that worked for him at the World Cup, but he only adopted it in March, almost as a last resort. Kevin Strootman, who had been key to his midfield, was badly injured in a friendly away to France in which Bruno Martins Indi was tormented by Karim Benzema.

Troubled by his defenders’ ability to handle one-on-one situations, Van Gaal went to watch three of them play for Feyenoord against PSV Eindhoven. They lined up in a 3-5-2, the extra man in the middle at the back meaning one-on-one situations were minimised, and that, he realised, was the answer. That the system was right for Holland can – if you judge on results, which not all the Dutch do – hardly be denied, but why did that make it right for Manchester United?

The reason a back three fell largely into disuse in the first decade of this century was the prevalence of teams playing with a lone forward. Deploying three defenders against two forwards meant – even in a zonal system – two markers plus a spare man. One marker plus two spare men was overkill: one of those spare men was redundant and effectively gave the opponent an extra man in midfield.

So, why at United? Why, almost before looking at his squad, while insisting every player started with a blank slate and he was using the pre-season tour of the US as a learning process, and with six weeks of the transfer window remaining to add reinforcements, did Van Gaal apparently decide he would stick with the 3-5-2 he had used in the World Cup?

It was not thoughtless idealism, sticking to a philosophy come what may; above all else, he is a pragmatist, and, besides, 3-5-2 only became his preferred shape in March.

Although he used a 4-4-2 at times with AZ and Bayern, Van Gaal has tended to prefer to use three men in central midfield. Even with a 4-4-2, he tended to have one forward – Thomas Müller or Ivica Olic – who could drop back to become an additional midfielder if required. Three in midfield, even if one of them is a slight luxury such as Wesley Sneijder, offers security and flexibility. Besides which, looking at United’s squad, Van Gaal cannot have seen any potential pairing that would prove adequate in central midfield – a problem that haunted David Moyes last season and was a concern long before that.

So he wanted three in midfield but how to do that when United’s obvious strength is the front two of Robin van Persie and Wayne Rooney? They may not necessarily be great as a partnership, but they are both, when fit, clearly among United’s four or five best players. Both have played wide in their careers, but neither enjoys it so to accommodate the pair, they have to be used as a central partnership. Perhaps in the long term there are better solutions, but there are other issues that require more immediate attention.

Rooney could be used as the central player of three in a 4-2-3-1, foraging back when required, but where does that then leave Juan Mata, Shinji Kagawa and Adnan Januzaj? That would leave United needing wingers – and Van Gaal has made clear he is not impressed with the options available. A 3-5-2, though, permits both a midfield three and a front two (the other option would be a 4-4-2 diamond, but United lack an obvious holding midfielder who could become a de facto third centre-back when the full-backs advance), even if it does require significant defensive readjustment – and it may be that Van Gaal underestimated the unfamiliarity of British defenders with the system.

Ángel di María is presumably seen as a player to operate – as he did last season – on the left of a midfield three.

Self-sacrificing and tactically intelligent, he was perfect for the role, going wide when Cristiano Ronaldo came infield, dropping back to cover when Marcelo or Fábio Coentrão charged forward. He is also a master at leading counterattacks, with a keen sense of when to carry the ball and when to play the pass. Without a wide player beyond him, his role will be different at United, but it is easy to imagine him covering for a wing-back’s foray or linking up with Rooney when he drifts left.

What is intriguing now is that Van Gaal, having decided early on 3-5-2, seems to be making signings to stick to it. Although Di María can play just as well in a 4-3-3, either as a midfielder or a forward, or in a 4‑2‑3‑1, it would seem odd if Marcos Rojo has not been signed as the left-sided of three centre-backs. Although he can operate at full-back, as he showed at the World Cup, it would seem strange for United to invest £16m in a player having already spent double that on another player, Luke Shaw, in the same position.

It may be, of course, that Rojo ends up playing at centre-back, or that he keeps Shaw out of the side. Van Gaal will do what it takes to win, even if that means ostracising the expensive or the high-profile. That’s the how. The why, though, is the combination of a recent convert to a different shape and his inheritance of a squad that was incomprehensibly unbalanced.