Moyes said: "I can remember when Rio Ferdinand went to United and people questioned him.
"But you only need to look at Ferdinand now to see how good a player he has become and what he offers United.
"I've no doubt that Joleon will do the same here at Everton because he is conscientious and there is nobody more determined than him to put things right.
"There has been maybe one good game followed by one bad game, but you can go through those periods.
"I was a defender myself, so I know that you can have spells where you make mistakes that turn out to be costly. There are big expectations on him and that happens.
"He was such a good player last year and he won all our player-of-the-year awards and was involved with England but there will be occasions when his form levels out."