The champions have played some exhilarating football up front but they have also been susceptible at the back, and the warning signs have been there that they could be handed the kind of hammering they've doled out to others already.
As far back as the middle of last month, Stats Zone analysis showed that they were allowing opponents "more shots this season than any other Premier League team" and that they had a "porous" defence regularly "allowing the opposition somewhere north of 20 shots on goal".
This has been particularly notable at home, where United have been unreasonably welcoming. Indeed, the 22 shots they allowed City to rain in yesterday was the same number they had conceded to earlier visitors Chelsea, whom they had beaten 3-1 despite having eight fewer shots than their guests.
Furthermore, Chelsea were the third set of visiting Londoners to get 20+ attempts at Old Trafford, following Tottenham (who were beaten 3-0) and Arsenal (hammered 8-2). We noted at the time that "Before this campaign, United had conceded 20 shots on just two occasions in five seasons", but they haven't tightened up considerably since: previous visitors Norwich managed 17 attempts and could easily have embarrassed the champions.
Clearly, this season United have been unusually generous as hosts, but their visitors have been unable to take advantage of their hospitality. Eventually, if they kept leaving the back door open, the champions were going to be ransacked by opportunists Ã¢ÂÂ and it so happened that it was the noisy neighbours.
And so to "the result that shakes the Premier League to its foundations", as one particularly overexcitable radio station declared it. However shocking or predictable, City's second-half spree certainly made the derby a game of two halves Ã¢ÂÂ which gives us chance to play with Stats Zone's ability to focus on specific times in the game. (Drag the sliders under the pitch to choose your timeframe, or just tap a half to focus on that 45-minute period.)
As they chased the game with increasing urgency, United's passing figures collapsed in the second half Ã¢ÂÂ while City's bloomed as they combined keepball with deadly penetration.
Note too how empty City's right-back area is, Micah Richards' aggressive attack-minded play Ã¢ÂÂ highlighted below by showing his passes received and made Ã¢ÂÂ pushing back Ashley Young and thus denying United one of this season's most fruitful creative outlets.
It's tempting to say the match hinged on Jonny Evans' 47th-minute red card, and it's foolish to deny its effects Ã¢ÂÂ and those of Fergie typically refusing to give up the ghost and throwing more players forward.
However, the preceding week at Anfield, the champions' 11 men had also faded after the break, completing just 118 passes. This continued a gradual but long fall from the 267 they had completed in the second half of their Arsenal annihilation in August.
That said, Evans' dismissal (and United's chasing of the game) certainly left them more open at the back. None of City's nine first-half crosses reached a man in blue, but they completed five out of eight in the second half Ã¢ÂÂ including two assists.
Even so, the first half had merely followed the season so far in that United had registered fewer shots on goal than their opponents; before the weekend they had conceded 146 shots while making 138 attempts of their own.
What changed was that for once, their foes were more efficient in converting shots into goals. Of City's seven first-half attempts, only one hit the target Ã¢ÂÂ Mario Balotelli's opening goal Ã¢ÂÂ while Joe Hart saved from Wayne Rooney and Anderson.
As expected United upped their efforts after half-time with seven shots but only two on target (including Darren Fletcher's pre-desolation consolation) Ã¢ÂÂ but so did City with 15 shots, although 10 of those came after Ferguson's desperate double substitution midway through the half.
In other words, Joe Hart saved three out of four shots on target while David de Gea could only stop one of City's seven. However, it would be very harsh to castigate the young Spaniard, who has been performing heroics for United this season: only Bolton (55) have conceded more shots on target than United (54).
Besides, the finger could equally point at the attackers: Nani, Young, Danny Welbeck and sub Javier Hernandez couldn't muster a single shot on target between them. So far this season, United's attacking prowess has seen them simply out-punch every opponent Ã¢ÂÂ but in City they came up against a team even more efficient and dangerous. As Stats Zone has demonstrated, it was only a matter of time before the champs were made to look like chumps.