The Northern Ireland international has filled the void left by captain Nemanja Vidic, who has been absent since November with a knee ligament injury.
"I must admit my heart sank a little when I learned just how long Nemanja Vidic was going to be out," Ferguson wrote in the club's match programme.
"He and Rio Ferdinand had become the cornerstone of our defence but Rio responded brilliantly, despite his back problem, and has now struck up a fine partnership with Jonny Evans.
"There is no doubt he has been helped playing alongside the experience of Rio and he is now arguably the best defender in the country."
Evans joined the Red Devils as a trainee.