Keane, now manager at Sunderland, has been taken aback by the 22-year-old's sensational impact on United's three-pronged assault for silverware.
Ex-skipper Keane told The Sunday Mirror: "I think Ronaldo is outstanding. If you had asked me two or three years ago I would have told you he could go on to be the best player in the world.
"I thought that would happen after he had been playing over a longer period of time, but his form this season has been a different class."
The Black Cats chief admitted that the winger's dazzling array of tricks make him one of the game's great entertainers.
"The skills Ronaldo has are frightening," said Keane. "He is an entertainer and fans at United love that. It's great to see and they are the type of players you would pay to watch."
With United six points clear at the Premiership summit, in the semi-finals of the FA Cup and quarter-finals of the Champions League, Keane can envisage a repeat of 1999's Treble.
"I am sure the Treble win of 1999 could be emulated," he said. "They are in good form - but they need luck, though. It'll be hard but I think an English team has a good chance (in the Champions League) this year."