Rooney had a disappointing World Cup for England and failed to find the target as they exited the tournament at the last 16 stage after being beaten 4-1 by Germany.
His form was in stark contrast to the previous domestic campaign when he scored 34 goals in all competitions for United.
But Ferguson expects his talismanic striker to kick on when the season opens in August.
Ferguson told ESPN: "He had an improvement in terms of his timing and movement in the penalty box. His positioning also improved from the previous season.
"He got us over 30 goals and if he does that again and adds a little bit to his game then that is what we expect.
"The boy has got an incredible armoury of talent and what we have to do is to wait for that maturity.
"We also do nurture it and try to advise and coach in a fashion which will improve his game. But the most important thing I think he needs now is maturity and, when he gets that, I think you will see the complete footballer."