Rio Ferdinand admits Manchester United boss Sir Alex Ferguson is trying to adjust the game of Wayne Rooney.

"The manager has asked him to be that player that runs through a lot more. He can do that," Ferdinand told PA Sport.

"With more time and the way he keeps working he can become more and more of a threat in behind. He has the strength and the nose where to run.

"Sometimes he needs to be more selfish. To score 25 or 30 goals a season you have to have a certain amount of selfishness. He probably needs a little bit more.

"He is capable of getting that number. I have played against him in training. I know how difficult he can be to play against and if he continues to keep working in the right areas he has the capability to be top scorer in the Premier League.

"He is playing more like a number nine for us and for me he could easily be the best No. 9 in the world given time," added Ferdinand.

"He has the capability and more tools than any other number nine in the world to do that. I have every confidence in him.

"It's just a matter of him being given the time and I am sure he will put it into practice."