The Dutchman's first-half hat-trick help United to a 3-0 win over Aston Villa on Monday night that saw them clinch their 20th Premier League title.
Ferguson insists that the 29-year-old, who has now scored 28 goals for the club since joining in the summer, has made a bigger mark on a title-winning season than any other player.
"I think Robin is such a great player for Arsenal and when great player became available and you feel United should have him you have to go for him," Ferguson told ESPN. "I think we had an expectation of him - his performance last season was incredible.
"I remember Arsene saying 'he's a better player than you think.' I knew he could gel into the team.
"He has been unbelievable. His early form that first six months was fantastic. He hit that spell where he hadn't scored a goal and every striker has a dry spell but when he scored that penalty kick at Stoke I thought, he's alive now and today he was unbelievable."
Van Persie moved to United from Arsenal for £24m in the summer.