Sir Alex Ferguson's unsettled back four is far from watertight in the absence of their injured captain Nemanja Vidic, writes Richard Jolly.
Manchester United's fragile defence miss their leader Nemanja Vidic
Comebacks can obscure the reasons they are required.
When they are as dramatic as a recovery from 3-0 down to procure a point at Chelsea and when they follow the familiar narrative of Manchester United's refusal to accept defeat, they can prompt more praise than a regulation victory.
Wayne Rooney's marauding bullishness, Javier Hernandez's predatory ebullience, Ryan Giggs's wonderful delivery, Paul Scholes's timelessly classy passing: all are deserving of acclaim. Yet the fact remains that United trailed 3-0 to the least distinguished Chelsea side of the Roman Abramovich era.
Sir Alex Ferguson argued that one was a magnificent strike - Juan Mata's volley - and the other two were own goals, even if David Luiz was credited with Chelsea's third, despite a hefty deflection off Rio Ferdinand.
His analysis is true up to a point.
Yet examine United's defensive record and it is decidedly mixed. This is the team with the most clean sheets in the Premier League. Yet they have also conceded six times to Manchester City and three each to Blackburn Rovers, Chelsea and Newcastle United. This is a side that veers between frugality and fragility.
When Nemanja Vidic has played, however, United have only been breached twice in six games and even those were the consequence of a goalkeeping error and a penalty that should not have been awarded. When the captain has been missing, they have let in 22 goals in 18 games.
Now the Serbian is sidelined for the season and, while every team would lament the loss of the best defender in the country, United miss his brand of defiance. They have been further hindered by the injuries that have made continuity impossible, yet each of the four other central defenders appears a superior player when Vidic is by his side.
In Chris Smalling's debut season at United last year, he looked assured alongside Vidic and inexperienced without him. Ferdinand's finest form of his career has come as the Serb's sleeker partner; without him, decline is more apparent.
For all his abundant promise, Phil Jones's best United displays have come at right-back or in midfield, not in the central defensive role he is likely to adopt in the long term.
And while his own goal was unfortunate, Jonny Evans lacks presence and is often error prone.
With Patrice Evra below par on the left, a rotating cast of right-backs and the uncertain, if occasionally brilliant, David de Gea behind them, the men in the middle merit sympathy. Their counterparts at other elite clubs may be protected better.
Of late and of necessity, Sir Alex Ferguson's midfield pairings, Michael Carrick alongside either Scholes or Giggs, have been essentially constructive. The advantages of a more destructive player were clear last month when Newcastle's enforcer Cheik Tiote turned in a performance of awesome power in their 3-0 defeat of United. Ferguson, however, has long been reluctant to sign such a midfielder.
There were hints that the defence needed rather more shielding when Ferguson played an ultra-defensive 4-5-1 at Liverpool in October. Otherwise, however, the manager's tactics get bolder as he grows older.
It provides an invitation to opponents, if they are brave enough to accept it.
As Blackburn, Newcastle, Chelsea and City have shown, attacking United can be a profitable policy. It is stretching it to say they have a soft underbelly but Ferguson's defence is far from watertight.
One of the transfer window's lower-profile signings appears among the most effective. Apart from in his native Iceland, or among supporters of Reading, Hoffenheim and Swansea City, few may have noticed Gylfi Sigurdsson's arrival at the Liberty Stadium.
Borrowed from the Bundesliga club, Sigurdsson played for Brendan Rodgers at Reading. Their reunion appears a masterstroke: the Icelander provided the pass for Danny Graham's winner against Arsenal, scored one and made one in Saturday's victory against West Bromwich Albion and has slotted in seamlessly to Swansea's passing game.
When, to Stoke City's fury, Robert Huth was dismissed in Saturday's defeat at the Briannia Stadium to Sunderland, he became the seventh player sent off by Martin Atkinson in the Premier League this season. He showed a further 11 red cards last year. Both totals are far higher than those of any other official.
It all suggests he is applying the laws very differently. That should be a worry.
More Premier League, s10-11