The United boss, who sold Ronaldo for a world-record £80million in 2009, declared that his former prodigy is even better than all-time greats Zinedine Zidane and Luis Figo, two of Real's most celebrated signings in recent years.
Ferguson said: "Florentino Perez had all the Galacticos at Real Madrid when he was president first time around. He had Figo, Zidane, Roberto Carlos, Raul in that spell. But I don't think any of those players were as good as Ronaldo.
"Zidane was a fantastic player, Figo was a fantastic player. But not as good as Ronaldo. So I think Mourinho's team are still living up to the reputation that Real Madrid can produce the top players.
"Ronaldo has beaten all the records there. He's beaten Di Stefano, Puskas, Hugo Sanchez. So obviously he is indelibly printed in their history now.
"You have to say he is pivotal to all they do - 179 goals in 178 matches tells its own story. I think I might have to think about blackmail. I might have to tell a true story about him!
"He's a fantastic player. I knew he would improve tremendously, he was still young when he left us."