The 70-year-old manager has stated that although the club weren't always in for the Dutch striker, circumstances changed to allow the move to go ahead
The 29-year-old Netherlands international officially moved to Old Trafford on Friday, and Sir Alex revealed that the club's interest was only turned into a formal approach once the Dutchman refused to sign a new deal in north London.
Van Persie was the Premier League's top scorer last season, after notching 30 goals and the 70-year-old Scot, believes the Dutchman's talents stand up against the world's best.
"Robin is a world-class striker with a proven record in England and in European football," Ferguson told reporters.
"His talents need no introduction to our fans. His movement, finishing and all-round ability are outstanding.
"This is the best league in the world. Messi wouldn’t score 72 goals in this league.
"We need a finished player. Scholes and Giggs are coming to the end and we need maturity. That's why Robin is very important now.
"We never thought we'd get Van Persie. I couldn't see us getting him, but when he refused a new contract, that's when we acted."
Ferguson also confirmed during the press conference that Van Persie was likely to be available for selection ahead of the club's first game of the season against Everton away at Goodison Park, but refused to disclose whether that would be from the start or as a substitute.