Manchester United boss Sir Alex Ferguson rates Robin van Persie as the buy of the season.

Ferguson insists Van Persie will leave his more expensive rivals in the shade and that he wouldn't trade the former Arsenal captain for all of Roberto Di Matteo's new arrivals at Chelsea.

The Scot said: "They have brought in Oscar and Hazard, two similar type players.

"They are adapting to having these players with (Juan) Mata also. They are in behind the front player all the time and it is not an easy system to deal with. We just have to find a way.

"But Robin van Persie's performances are getting better all the time now. He has got there with his fitness. Last week against Stoke and on Tuesday against Braga his movement was fantastic.

"I really was impressed with that. I thought it was really top, top stuff. Hopefully it does make a difference. We bought him for that reason - to make a difference.

"We brought him in to give us maturity up front and also the flexibility of different types of players. With (Danny) Welbeck, Chicharito (Javier ­Hernandez), Van Persie and (Wayne) Rooney we have fantastic alternatives and that could make the difference, the four of them.

"There is a lot of ways a league can be decided. The way we lost it last season is one way, on goal difference. It was unusual for us. It is very, very difficult bedding new players in.

"The thing at a club like ours, and any big club, is that you don't stand still - you have to move on every year.

"When we realised Van Persie was not going to sign for Arsenal, we did our very, very best to get him at our club. We are delighted. The signs have been terrific. He is a great professional. He trains very well and he has made an impact immediately."