Fergie sees Santa Cruz as the big man who Cristiano Ronaldo and Wayne Rooney can feed off.
Rovers will be desperate to keep their main striker, but with manager Mark Hughes on the way to Manchester City they will struggle to hold their prize asset.
Ferguson once said of Santa Cruz: "I'm not sure how much he cost but he has proved great value."
He is on a four-year Blackburn contract worth around £40,000 a week but United could double that.
Santa Cruz is also close friends with former Bayern teammate and United midfielder Owen Hargreaves.