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Manchester United need to improve in central defense—it's a position which has cost them dearly this season.
Their two starting centre-backs against City each had their own hamartia—one was too slow, and the other too inexperienced.
Rio Ferdinand struggled against David Silva, Yaya Toure and Carlos Tevez, while Chris Smalling was caught out a few times and was at fault for the goal—his failure to mark or even spot the run of Vincent Kompany being the main difference in the match.
And talking about the basics, their failure to even make many effective clearances could've also cost them goals if it wasn't for better final third passing and finishing.
Nemanja Vidic, with his great man-marking ability, brilliant clearances and robust, accurate tackling was sorely missed at Eastlands—like he has been all season.
However, with there no guarantee he'll return the same player, and a lack of depth at centre-back a clear problem—with the error-prone Jonny Evans the only other experienced option—something needs to change in that area.
A new signing could be a solution, such as Mehdi Benatia or Dedé, or bringing in a defensive coach (or get a better defensive coach if they already have one).
Or a mixture of both would work, as United need more pace at the heart of defense, and also need to tighten up on their man-marking and positioning.