This season has been one of the club's worst ever for its defense.
The Red Devils' back four look a completely different side from the one that once helped Edwin Van Der Sar to a record number of clean sheets.
United's 6-1 defeat at the hands of Manchester City is a testament to this, and really when you consider just how bad the back four have been this season, it's no surprise that United were knocked out of the Champions League.
First off, Patrice Evra has been poor. Very, very poor.
The extra attention on him due to the Suarez controversy has not helped the Frenchmen at all this season, and he continues to expose his fellow defenders in crucial situations this season, as Kuyt's goal at Anfield demonstrated.
Evra just doesn't demonstrate the same eagerness and drive that he used to for the club, which is a big problem when you consider that he's United's captain. Against Chelsea, the Frenchmen was poor, didn't seem to concentrate well and often looked fragile in front of Chelsea's pacy attack.
Rio Ferdinand and Jonny Evans, however, deserve the brunt of the criticism.
If I had a penny for every time Ferdinand loses concentration and jeopardizes his own goal, I would have one heavy, jingling wallet.
In fact, I would be ever richer if I had one for every time Jonny Evans is just standing around, completely unaware of the opponent striker's presence. Ferdinand is looking to be far past his prime, and Jonny Evans doesn't ever look like hitting his prime.
These two are one of the main reasons why de Gea has had such a hard time, as they both have failed to produce the kind of solid fortification that a young goalkeeper like him needs.
One of the main reasons why United's back four have been so poor is, of course, the absence of Vidic.
Without the Serbian, there seems to be no sense of communication and strength that is needed in a team like United's defense. They often look too fragile in front of their opponents, and as Chelsea showed the other day, they absolutely crumble under pressure.
The club's injury crisis definitely played a part in the back four's jadedness, as it proved that United were rather limited in the defensive area. Perhaps Ferguson should have never sold Wes Brown and John O'Shea, as that kind of experience and stability could really be needed right now.
Though, the young but talented Phil Jones and Chris Smalling are beginning to return to full fitness.