Liverpool exploited Manchester United's soft underbelly at Anfield in emphatic style. It is an area the Premier League leaders need to improve, fast.
Liverpool expose Man United's leaky defence
For an instinctively attacking manager, Sir Alex Ferguson has always had a carefully disguised sense of caution. His great teams have always been built from the back, constructed around men whose names go together as often as Ferguson and victory.
There was Willie Miller and Alex McLeish at Aberdeen, then Steve Bruce and Gary Pallister, Jaap Stam and Ronny Johnsen and now Rio Ferdinand and Nemanja Vidic at Manchester United.
The list of great defensive duos need not be expanded. On this evidence, Chris Smalling and Wes Brown are not pressing for immediate inclusion.
The callow Smalling has rare promise, but Brown possessed that once, too: now the man Ferguson once described as "the best natural defender in England" is a player whose season peaked against non-league Crawley Town in the FA Cup and who looked uneasy at Anfield. The unimpressive understudies may be convenient fall guys, but they represent a legitimate reason for United's defeat.
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Defending is about anticipation, about seeing danger when none is apparent. It would be admittedly harsh to fault either for Dirk Kuyt's second goal, caused by Nani's aberration, but the Dutchman materialised unchecked in the six-yard box on a hat-trick of occasions.
That would not normally happen: Ferdinand's ego can obscure his ability to read a game while Vidic's unofficial motto is "thou shalt not pass". He contemplates any means, fair or foul, in his bid; an attitude that ensured he was suspended for yesterday's game.
Minus the banned Vidic and the injured Ferdinand, this is a United side with a soft underbelly. Perhaps that accounted for Kenny Dalglish's boldness.
While shrewd pragmatism characterised Liverpool's win over Chelsea at Stamford Bridge earlier this season, a spirit of gung-ho rang through this victory. Dalglish sniffed a weakness and his marauding team exploited it.
Two strikers were selected, a sign that United's frailty was being targeted. Kuyt and Luis Suarez were the outstanding individuals on show.
That reflects upon Smalling and Brown, yet an untried combination received insufficient protection. Michael Carrick and Paul Scholes form a flawed partnership in midfield. Deep-lying but not altogether defensive, they have a shared lack of pace. Scholes's greatness is established but, while the first half contained some magical passes, his 36-year-old legs mean he requires protection.
Carrick is increasingly unable to provide it; just as United were overrun in the closing stages of their defeat at Chelsea on Tuesday, they encountered fitter, faster opponents in the shape of Steven Gerrard and Lucas.
Circumstances might have forced Ferguson into a show of daring, with two wingers flanking a pair of strikers. But the bite of the injured Brazilian Anderson was missed along with the energy of the ill Darren Fletcher. No enforcer dovetailed with the two passers.
Yet without the big-game battlers, Ferguson's reluctance to field the three-man central midfield he favours in the major games highlighted the continuing doubts about Darron Gibson's suitability.
Despite the depth of United's squad, it contains too many men of questionable pedigree. Theirs is a title challenge that has been built around a small core; the premier pensioners, Ryan Giggs and Scholes, the defiant defenders, Vidic and Ferdinand and their reliable ally, Edwin van der Sar, plus the flamboyant flair player Nani.
Yet a second defeat in a week suggested United are rattled. Games have been lost along with their composure.
Nani's attempts to get Jamie Carragher sent off were disdainful, even if the deep gash in his leg proved there were grounds to his argument, before exiting in tears and when Van der Sar, a 40-year-old with a reputation for calmness runs 70 yards to become embroiled in a melee, it is both uncharacteristic and extraordinary.
Another of United's more assured and eloquent individuals, their chief executive David Gill, is understood to have confronted the head of referees, Mike Riley, at half time after Carragher was only cautioned (so, too, for a challenge that was equally as vicious, was Rafael da Silva).
Ferguson, meanwhile, is boycotting the media, the sort of action that fuels the theory he has a persecution complex.
While he retreats to his metaphorical bunker to plot the downfall of enemies, real and imagined, Liverpool are reliant upon Arsenal to preserve a share of their record.
United retain the advantage in their quest for a historic 19th title but the danger is that the cost of a damaging few days will extend far beyond bruised pride.