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 skills Ronaldo has are frightening,' he added. 'He is an entertainer and fans at United love that.
'It is great to see and they are the type of players you would pay to watch.'