For Chelsea, in their unexpectedly successful start to the season, the new attacking midfield trio of Juan Mata, Oscar and Eden Hazard are inclined and encouraged to interchange, but seem to have realised that they cannot all occupy the middle ground. In recent games, Mata and Hazard, while often switching flanks, have been more disciplined in staying wide.
United, however, who would normally have matched up in an identical 4-2-3-1 formation, have for at least two of their past four games pulled in their wings, using players such as Tom Cleverley and Shinji Kagawa (now injured) instead of Antonio Valencia, Nani and the fit-again Ashley Young.
"If it turns out we play the diamond consistently it would be revolutionary because it is going against our history," Sir Alex Ferguson admitted in midweek. His argument was that "making yourself unpredictable" was invaluable, but in the first half against Braga attacks were all too easily anticipated without the required width as the visitors were able to sit back and block up the centre after stealing an early two-goal lead. Only after replacing Kagawa with Nani and moving Wayne Rooney to the left side were United able to recover and win the game with three goals from crosses.
Key to all this is Rooney's role. "I'm happy to play where the manager wants to play me," he said diplomatically after Tuesday's game. "We have a lot of top strikers and midfielders, so if the manager wants to play me in midfield or as a striker I'm happy to do that.
"I don't know what position I will end up playing. You don't know how things are going to develop. Obviously I have been a centre-forward all my career, so playing in midfield is different, but it's good to get that experience as well."
Once Rooney had recovered from the gashed leg sustained early in the season and was able to play just behind Robin van Persie, with two from Valencia, Nani and Danny Welbeck wider, United looked to have found a happy combination, albeit one that squeezed out the talented Kagawa.
Two problems showed up against Braga: Rooney, made captain because Rio Ferdinand was rested ahead of today's game, took on too much responsibility, often dropping too deep; then Ferguson admitted that Javier Hernandez had "given me a problem" by scoring twice on only his fourth start this season. When the system went to 4-2-3-1 for the second half, it was Hernandez who played as main striker, with Van Persie behind him. Rooney, meanwhile, was shunted to the left, where for either United or England he can sometimes appear peripheral.
Ferguson was amused to hear "an 80-year-old woman" ringing MUTV last week to announce that she was fed up with his diamond formation. "I'd better not play it, better not upset her," he said on Friday. Out of the mouth of babes and elderly ladies...
Which ever way the manager deploys his attacking forces this afternoon, he can only hope his defence tightens up with the return of Ferdinand and Patrice Evra. As Ryan Giggs put it: "We know the secret of winning leagues and trophies is to win games 1-0 on a consistent basis. Clean sheets are very important."
As for repeatedly conceding the first goal and occasionally the second as well he added: "There's two ways of looking at it. On one hand the lads have showed great character coming back in games and great quality with the goals they have scored. But you can't go through a season giving teams one or two-goal leads. We know it's something we've got to address and we will do."
Either Giggs or Paul Scholes is likely to start, though not both, for Ferguson needs to match the youthful energy of a Chelsea midfield which has impressed him as much as anyone by knitting together so quickly.
"They give real problems for defenders, where they're going to appear next, because they're all flexible players," he said. "If you can contain the three in the middle of the park then you've got a chance, but that's not certain, because their record so far has been unblemished and you've got to give credit for that.
"It's bold to play that way, for Roberto Di Matteo as a new manager for them. They had the confidence from winning the Champions' League, but he's been bold enough to do it and you've got to give the lad credit. I think it's very positive."
The way United can take advantage is with the sort of swift breaks that helped Arsenal win 5-3 at Stamford Bridge exactly a year ago, when Van Persie scored a hat-trick. "I remember watching the game, it was all counterattacks," said Ferguson, who will have filed the information away in that formidable memory.
The pace of Valencia down one flank could be invaluable in those circumstances as Chelsea's full-backs push forward, and Welbeck, who played no part against Braga, will hope he is trusted on the other side for such an important occasion.
The other option, even bolder, would be to sit Rooney behind both Van Persie and Hernandez, but that risks being outnumbered further back and would mean a temptation to revert to the diamond.
What is clear is that United cannot afford to get it wrong. Defeat would mean lagging seven points behind the leaders after a game that cannot be regarded as decisive at this time of season but may come to look that way once the points are added up in the middle of May.
"I'm looking forward to it," said Rooney. He is hardly alone in that.
Chelsea (4-2-3-1): Cech; Ivanovic, Cahill, Luiz, Cole; Mikel, Ramires; Mata, Oscar, Hazard; Torres.
Manchester United (4-2-3-1): De Gea; Rafael, Ferdinand, Evans, Evra; Carrick, Scholes; Valencia, Rooney, Welbeck; Van Persie.
United's Capital gains in London
Today's game is Manchester United's first this season in London, where they have not lost since a 1-0 defeat by Arsenal in May 2011, remaining unbeaten in the capital throughout last season. Chelsea will be only too aware of what happened in the Premier League fixture at Stamford Bridge last season, when they raced into a 3-0 lead only to see United stage a storming comeback to earn a 3-3 draw.
4 March 2012 Spurs 3-1 (Young 2, Rooney)
5 February 2012 Chelsea 3-3 (Rooney 2 pens, Hernandez)
22 January 2012 Arsenal 2-1 (Valencia, Welbeck)
21 December 2011 Fulham 5-0 (Welbeck, Nani, Giggs, Rooney, Berbatov)
18 December 2011 QPR 2-0 (Rooney, Berbatov)
Chelsea v Manchester United is on Sky Sports 1 today, kick-off 4pm