Graphic: Manchester United's best defence

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The chart above details how well each of Manchester United's key defenders perform as part of a three-man unit.  These units - goalkeeper plus two centre-backs - were tracked for minutes played together, goals conceded and goals scored over the season so far.

Should a player be farther right on the X-axis, United scores goals more regularly (per 90 minutes) while he is on the field; the Y-axis indicates how regularly they concede while that player is in a key defensive position.

While sample size for some players is small - Michael Carrick has a total of 360 minutes at centre-back this season, Scott Wootton 281, Phil Jones and Michael Keane nominally 180 each - their positions above represent not so much a change in defensive efficiency but in United's tactics.

United is blessed with five players with whom they can be comfortable at the heart of defence, but problems emerge when they employ their sixth choice.  That guy, Carrick, is a central midfielder and part of a three-man central defensive unit concedes a relatively high average of 1.5 goals per 90 minutes - no matter who his partners have been.  What makes this worse is United feel the need to cover more for him and drop drop midfielders back to cover, creating a double-edged sword in which they concede more and score fewer.

Also noticeable is that Nemanja Vidic's presence almost automatically means United are entering a game with a more defensive mindset.  The Serbian's figures correspond to a significant reduction in both goals allowed and scored per 90 minutes - highlighting a change in Ferguson's tactical outlook.