Serginho played with Rodrygo at Santos and is set to face Real in the Club World Cup. Rodrygo will join Real next summer.
He told AS: "I know him well. He's four years younger than me but I played with him at Santos. He is another out of the youth team, fast, skilled, intelligent.
"Surely he will work in Madrid, but needs experience."
On facing Real, Serginho adds: "I do not say anything new. Year after year they're the best. I always followed them, especially since the time of Ronaldo ... but Ronaldo O Fenómeno.
"Cristiano Ronaldo is not bad either (laughs)."