It is a measure of Louis van Gaal’s vast self-belief that the new three-man Manchester United defence which he will introduce to competitive football against Swansea City at Old Trafford today is a system which has never won the Premier League.

The formation has popped up intermittently over the years. Liverpool’s Roy Evans was the last manager to be so bold in its introduction as Van Gaal when making a three-man unit out of Neil Ruddock, Phil Babb and John Scales in his first full season in charge, 1994-95. More recently, Roberto Martinez’s Wigan and Gus Poyet’s Sunderland used it as a defensive weapon which morphs rapidly to 5-3-2.

Van Gaal’s motive is more elaborate. He says his 3-4-1-2 allows him to deploy both a No 10 (Juan Mata) and two strikers, having found his squad “imbalanced” by the number of attackers he inherited. That trio play ahead of two central midfielders and two wing-backs – the lynchpins of the system, who need to attack and defend in equal measure.

There are risks attached. A couple of attacking full-backs in the opposition can have the space to damage this system down the flanks. A side which employs such a three-man back line can also be susceptible to rapid counter-attacks. But many central defenders will tell you that playing as part of a back three, rather than a four – as Southampton under Ronald Koeman and Harry Redknapp’s Queen’s Park Rangers may also do this season – is more comfortable.

One of the fundamental mechanisms underpinning that three-man line is the need for one of the defenders to sweep behind two who mark. It means a defender can follow a centre-forward into the opposition half, keeping tight on him to prevent the striker turning and running at the back line, safe in the knowledge that the sweeper is behind him, covering.

“It’s easier – far easier – in a three,” said Danny Higginbotham, the former Stoke and Derby defender who began his career at Old Trafford and is building a strong reputation as an analyst. “As a centre-back in a back four you have no safety net behind you. You’re always wary of a striker’s pace in central defence. If you go forward and get too tight on your striker you are vulnerable to the ball over the top. If you don’t go tight then they can turn and get the ball and run at you. But with this formation you can go and get as tight as you want. If the ball is played over the top, a striker may be quicker than you in the foot race but you know you’ve got the sweeper behind who has got two or three yards on it to cover him.”

One of the three will generally be designated the sweeper. In the three-man unit Higginbotham operated within at Derby under Jim Smith’s management when he moved there in 2000, it was Horacio Carbonari or maverick Nigerian Taribo West. For United in pre-season, it has often been Phil Jones, with Van Gaal’s lack of left-footed central defenders generally forcing him to play Jonny Evans on that side.

But the sweeping role can rotate, with the left-side defender doing the job if an attack is being mounted down the right and the right-sided one providing the cover for a raid down the opposite flank. Communication is a fundamental requirement, even more so than in a four-man defensive line.

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“If you have a sweeper he has to be the dominant one,” said Higginbotham. “He is behind so he can see everything and he should be dictating to the centre back pair – ‘You go there and you go there; I’m dropping five yards.’ The sweeper has to dictate the line as well, because there’s no point two pushing up when the opposition have the ball and one staying back, playing the opposition onside.”

United’s defenders are being asked by Van Gaal to develop a far greater spatial awareness: an understanding of where you stand in relation to the other two. But the manager is also asking one of the trio to be a ball-player – to step out and advance deep into opposition territory, with the security of that cover behind him. The system is about helping United to win the midfield by getting more players into it.

We have already witnessed Evans – the most comfortable on the ball of United’s defenders – going forward far more in the past month. That advance leaves an opposition midfielder with a difficult decision to make: whether to leave his station to pick him up or let him run.

“If he leaves home and goes to the centre-back and engages him, he leaves another man free,” said Higginbotham. “But if he doesn’t, he allows him right into his own half of the pitch. The whole picture can open up then. A midfielder is going to have to make a move and you might then find that Wayne Rooney, Juan Mata or Ander Herrera has dropped off deep into space, and is saying; ‘I’m free here; give the ball.’”

Thomas Vermaelen and Mats Hummels have been transfer targets for Van Gaal because they are ball-playing centre-halves with the left-footed bias currently lacking in United’s ranks. Their failure to capture either could load the odds against the system working, but against Roma in pre-season Jones’ unopposed advance saw United earn a penalty by winning the numbers game in midfield.

The system entails condensing the opposition’s space by advancing in three rows and pressing the ball high when without possession, while making the pitch as big as possible while on the ball. Liverpool’s Brendan Rodgers says it helps him get more men forward. But detractors believe the line of three, if not moving together, can be vulnerable to clever opposing strikers beating the offside trap, while Arsène Wenger claims that pressurising the opposition and creating more chances entails having three in midfield and three up front.

Systems, of course, are only as good as the players operating within them and Van Gaal will be taking encouragement from the way Evans, Jones and Chris Smalling have adapted. But an injury to Evans has been compounded by the hamstring strain that will keep Luke Shaw out for a month. In Shaw’s place few expected United to begin their fightback with Ashley Young protecting the left side of defence, while young Tyler Blackett may be promoted to fill the gap left by Evans. Those bastions of United’s defence, Rio Ferdinand, Nemanja Vidic and Patrice Evra, are gone and it will be a baptism of fire for the 3-4-1-2. Van Gaal will tell you it is all going to work out fine. Let us see.