The pair were teammates in Italy before Rudiger left last summer for the Blues.
"He's a player who did very well at Roma, especially last season," said Rudiger.
"We played in Rome with the same system as we do here, so he was the left wing-back. Most of the time I was playing on the left of the back three so I was next to him, and he was a big help.
"He's good at the offensive part, and also good at the defensive part. He is not afraid to make tackles, and he can play one against one because he's physically good. He also has good pace. For me he is a very good young player with potential."
Emerson admits he spoke with Rudiger before making the decision to sign for Chelsea.
And the Germany international told chelseafc.com: "I could have said whatever I wanted, but he had to listen to his feelings. I told him that maybe this chance comes only one time in life.
"He is a very, very humble guy," Rudiger adds. "To help him is a duty for me. Me and him were very close in Rome, and I want it to be the same here. I want to be like a big brother for him."