The Red Devils played a friendly against Sporting Lisbon in 2003, and Ronaldo was so impressive they immediately tried to sign him.
He went on to win the Premier League on three occasions and the Champions League before moving to Real Madrid in 2009.
"The first time I came across him (Ronaldo) was in 2003, when we played a friendly at Sporting," Silvestre told FourFourTwo.
"In the team talk beforehand, (Alex) Ferguson mentioned a talented young lad to watch out for. He didn't mention his name, but we all knew it afterwards!
"He was torturing poor John O'Shea on the left wing, showing off his speed and changes of direction with the ball. We were like, 'Hmm, it would be good if this guy would wear a United shirt instead…'
"After the match, Gary Neville came into the dressing room and said, 'Sign this kid, gaffer.' Sir Alex just said, 'We'll see what we can do.'
"I can remember sitting on the team bus waiting to leave, and we were there for quite a bit longer than usual because Sir Alex was speaking to Cristiano and Sporting, convincing him to come to Old Trafford."