This is shaping up as a make-or-break weekend in the two big battles at the upper end of the English Premier League as four of the top five go head-to-head in what could turn out to be the defining clashes of the season.
There is a small advantage for leaders Chelsea as they visit Tottenham Hotspur in the late kick-off, by which time they will know the result of the earlier Manchester derby. If United fail to win at City, Chelsea will be able to calculate whether they can play just to replicate United's result, knowing they will stay at least four points clear at the top of the table if they do so.
There is a similar advantage for Tottenham, who will need only a point to move into fourth place if City lose to United and will at least be certain whether a win will take them above City.
On paper, tonight's games are the most difficult left for Chelsea and United, even though they still have to visit Liverpool and host Tottenham respectively, and there is a strong probability that Chelsea will go on to win the title unless there is a shift in United's favour tonight. That shift almost certainly will have to be a United win and a Chelsea defeat, which would close the gap to just a point with three games to play.
The momentum is firmly with Chelsea, who have won five and drawn one of their six domestic fixtures since their Champions League exit against Inter. That has helped them raise the bar in terms of what will be needed to win the title and the latest estimate is that they will finish on 85 or 86 points, which is at the upper limit of what United can achieve, or even a little beyond it.
As long as they don't lose to Tottenham, Chelsea's ability to reach a total in the mid-80s won't be significantly affected, though they will be more wary after Arsenal's midweek defeat at White Hart Lane. Tottenham's players have certainly shed the inferiority complex that once pervaded this fixture, having won two and drawn one of their past three home league meetings with Chelsea, and they have good momentum, too, as they rank second to Chelsea over the past six league games.
Before Tottenham started to fight back in this fixture, Chelsea had won nine and drawn five of their 14 Premier League visits to Tottenham, and the Blues are not the only big team to have found White Hart Lane a tougher venue in recent seasons. Arsenal have won on only one of their past five visits, United have failed to win on two of their past three and Liverpool have lost at the Lane for the past two seasons.
Those results give Tottenham hope of the result they need tonight to keep them in the race for fourth place, and their home form is certainly good enough to make them a credible Champions League team, as their tally of 38 points from 17 home games is only four or five short of being a top-class figure.
What has dragged that total down is not their performance level against the top teams - they have won three of four at home to other teams in the top six - but a couple of surprise defeats against lesser teams, Stoke and Wolves.
The key factor in those defeats was that Tottenham could find no way through the packed away defences - both losses were 1-0 - and their record at home, when scoring, is exceptional (their only failure in that scenario being the 3-1 defeat by Manchester United in September).
Notably, the three best away defences to have visited White Hart Lane this season have all taken something. United and Stoke (both 16 goals conceded) won, while Aston Villa (19 goals conceded) had a goalless draw. That indicates difficulty against Chelsea (joint-best away defence with United and Stoke), but Tottenham's scoring record is good enough to make them worth a risk at tonight's odds, which underestimate their ability to win.
City v United is difficult, with the hosts in good scoring form and United struggling as they await Wayne Rooney's return to full fitness. Rooney is expected to start tonight, but his ability to last 90 minutes is unproven, after he played less than an hour against Bayern Munich 10 days ago, and United's lack of a cutting edge without him was evident in last week's goalless draw at Blackburn.
City's potent attack threatened to outscore United in the reverse fixture, but their defence, then as now, can let them down. The cracks were not exposed in their heavy wins against Birmingham City, Burnley and Wigan, but it was a different story in their most recent game against high-class opposition when they lost 2-0 at home to Everton.
City have had it pretty easy under Roberto Mancini, playing only four games against top-eight teams, but their record in those four games suggests they are still some way short of where the club's super-rich owners would like them to be, as their only win was the 4-2 victory at Chelsea, which came after the hosts had dominated the first half and then self-destructed after the break.
As with Spurs, City's chances rest on their ability to score and the odds aren't too attractive, unless Rooney doesn't start.
Years since the last draw in a Manchester derby hosted by City: 17
Wins out of 13 at home for Tottenham when they have scored: 12
Shortlist: Fulham, Stoke, Cardiff, Leeds, Auxerre, Getafe, Bilbao, Mallorca.
Top five bets
1 Fulham home win
Still remarkably good value at home to teams below them on the table
2 Leeds away win
Back on track for promotion and can score fourth straight win
3 Getafe on handicap
Only defeat in past eight games was against Real Madrid
4 Bilbao home win
Have dropped only two points from their past 24 at home
5 Mallorca home win
The best home team in Spain bar the top two in the table