Ferguson spoke to his protege in Manchester ahead of City's encounter with Real in November, then went to the Etihad Stadium to see the winger take the Blues apart in the opening stages of what ended up being a 1-1 draw.
And the Scot has no doubt the improvement he oversaw during Ronaldo's six-year stint at United has continued since his arrival in Spain.
"Maturity brings many things," Ferguson told Inside United.
"When I went to see them play against City, some of his decision-making in terms of passing was brilliant. One-touch passing, good crosses.
"In the six years we had him, you just saw his game grow all the time, and he was a fantastic player.
"Now you see the complete player.
"His decision-making, his maturity, his experience, plus all the great skills he has got, they all make him the complete player."