United landed Rojo for £16million from Sporting Lisbon this summer, with Nani going the other way on a season-long loan.
The move eventually went through despite a third-party ownership dispute with Doyen Sports holding up the deal.
And Ferguson has backed Louis van Gaal's pursuit of the 24-year-old.
"Rojo, the Argentinian boy, I watched him at the World Cup," he told MUTV.
"I thought he did really well for them and I think he's capable of playing either left side as a wing-back if you like or as a centre-back."