1. What shape will the midfield battle take?
Sir Alex Ferguson has caused quite the stir by fielding a diamond formation that narrows the Manchester United midfield to thoroughly unfamiliar dimensions. "The level of the game in England and Europe now is so high that making yourself unpredictable is going to be a strength," Ferguson has explained. "Teams will have to think whether we're going to use two out wide or the diamond, because we have players capable of doing both."
United's midfield may look more cubic zirconia than diamond, with Shinji Kagawa out for a month with a knee injury and suspicions that Darren Fletcher might not be fit enough to anchor the midfield after playing 90 minutes against Braga on Tuesday evening (do his two full matches in five days for Scotland earlier this month make a better argument for or against?). But 4-4-2 carries a risk against Chelsea's midfield. Ferguson said Chelsea always plays through midfield as if that made things easier for his men, but it will take some stifling.
"Now you tend to be facing more Barcelona-style players -- the small, intricate ones who get in the pockets behind you and play the killer pass," Fletcher said of Chelsea's change since the arrival of Oscar and Eden Hazard alongside Juan Mata. "You have to be aware, look over your shoulder and communicate. [But] if we do win the ball, it will leave a lot more spaces than you normally expect against Chelsea."
Roberto di Matteo's side has certainly looked more vulnerable this season, but maybe too little has been said about the work done by Ramires and John Obi Mikel in Chelsea's other midfield unit.
2. Are we in for a high-scoring thriller?
There's no denying that both of these teams look more porous than usual. It's near impossible to imagine Chelsea conceding two goals to both Spurs and Reading in the Jose Mourinho era against which it is rightly and wrongly held up. United, plagued by injuries in defense, has let in two or more goals in half its league games so far this season. Both teams conceded twice in this week's Champions League fixtures.
"So many times, the offensive players didn't come back to help the defense; they usually only had the four defenders and one midfielder in front of the box," said Fernandinho, who scored Shakhtar Donetsk's winner. "We already knew [Chelsea's] strengths and weaknesses; we played as we wanted to. When we won the ball we moved it quickly over the ground and trusted our counterattacks to hurt them."
"United is a similar team, playing on the counterattack," Chelsea goalkeeper Petr Cech admitted, who was responsible for keeping the midweek scoreline respectable. "They will play more or less the same way as Shakhtar. But United has been conceding and leaking goals as well, so it will be very exciting." It's not clear which United goalkeeper will face Chelsea, with Anders Lindegaard reported fit on Friday following a thumb injury, and David de Gea (who made a couple of brilliant saves at Stamford Bridge last season) reported in Spain to be unhappy playing in England.
3. Who will start up front for Manchester United?
The inconsistent Fernando Torres still divides opinion amongst Chelsea supporters, but we should not expect Roberto di Matteo to spring a surprise at Stamford Bridge on Sunday. For Manchester United there is more choice: last weekend at Stoke, Ferguson started with three forwards on the pitch (Wayne Rooney, Robin van Persie and Danny Welbeck) and Javier Hernandez on the bench; it was the ceaseless momentum of Hernandez that rescued United against Braga.
So who'll get the nod? The link up between Rooney and van Persie is looking as good now as it did on paper. "Playing against them any time is difficult," Stoke City goalkeeper Asmir Begovic said, who had to pick the ball out of his net four times, with three goals coming from Rooney and van Persie. "But now it looks like they are getting their mojo together and gelling. Any mistake we made was punished by them." Ferguson felt the same, saying, "The kind of movement we saw between Wayne and Robin bothers teams."
It is patronizing to say simply that Welbeck is the sort of player who seems always to make a nuisance of himself, given how nicely he also found a rhythm with van Persie against Stoke, but it is also fair to say that Hernandez in the kind of form he showed earlier this week probably carries with him a greater sense of inevitability in the final third. Ferguson praised Welbeck as well as Rooney and van Persie following victory against Stoke, but if Hernandez has played his way into the starting line up, it will most likely be in Welbeck's place.
4. First goal wins?
It is often said that big games such as this hinge on the first goal, but that doesn't feel right on this occasion. Last season at Stamford Bridge, Chelsea went 3-0 up with a David Luiz goal not long after the half-time break as United struggled -- "this was shaping up to be remembered as a collective loss of nerve", said the Guardian's match report. Yet the game ended 3-3 thanks to two penalties on Rooney and a late Hernandez header. United has gone behind in eight of its 12 games so far this season, but lost only two of them (away to Everton and home to Spurs).
"Obviously we are pleased that we are able to come back and show great character," said Ryan Giggs. "But you cannot go through a season keep going behind and having to rely on the lads up front to score goals. Against Chelsea if we go behind it will be tough to come back." Especially as Chelsea has pulled off several comebacks of its own, recovering from losing positions against Reading, Norwich and Tottenham to collect nine points.
5. What will this match do to the title race?
This is already being billed as one of the season's most significant games, if not a title decider. It's true that the best teams recognize the value of points won early in the season, but also, titles contenders are usually divided by their records against the rest of the league, particularly those frantically treading water in the deep end. Still, Chelsea (22 points) has opened up a four-point gap on second-place United (18 points, level with Manchester City, which will take second if it beats Swansea City on Saturday).
You may already have read that so far this season Chelsea is 11 points better off in like-for-like fixtures than last; City has collected two more than from the same fixtures last season, while United is a point worse off. Whether these stats really mean anything is a matter for debate (Chelsea is not the only team to have changed over the summer, and several of its opponents lost key players in that spell) but even a nascent league table is something tangible for both sets of fans to work with. The Blues will remain top of the table no matter what happens this weekend, but a win on Sunday would keep it at least four points, and as much as seven points, clear of the Manchester clubs. Defeat to United could potentially leave all three teams just a point apart.