On Tuesday, the Portuguese attacker will run out on the Old Trafford turf for the first time since making the 93 million euro switch to Estadio Santiago Bernabeu back in 2009.
When asked whether the 28-year-old Ronaldo has since established himself as the best player in the world, Ferguson told ITV: "At this moment in time I think he is."
"Ronaldo's a special player, we know all about that.
"He's a mature man now. I think he'll get a great reception, but he's got a job to do - he plays for Real Madrid now - but we'll do our best."
The Scot also agreed with Jose Mourinho's declaration that the visitors will score at Old Trafford, adding: "I think they will. But so will we."
United holds a slight edge after securing an away goal in the two sides' 1-1 draw in the first leg.