Fonte was 17-years-old when he started playing and training with 16-year-old Ronaldo at Sporting Lisbon, before the winger moved onto Manchester United.
Fonte said: "Ability-wise he's up there with the best I must say. His technique, creativity and vision of the game and understanding are better than everyone else in the team at the moment.
"He gives us something else. Creativity-wise Ramirez is up there, no doubt about it.
"Listen, you can have all the talent in the world but if you don't work sooner or later you're going to start going down in your career.
"You have to maintain that and keep improving. And that's what Ronaldo did. That's what I try to pass on the message, for example Ramirez is 22 he knows if he wants to go even higher he has to keep working.
"He has the talent but it doesn't mean anything if you don't work."