The pair were together at Bolton Wanderers.
"Marcos was just 19 then [at Bolton] and he was almost like Nathaniel Chalobah or the other guys here now who are coming into the team and trying to break through," Cahill told Chelsea's official website.
"He was in that position at Bolton and he is big and tall and strong physically but I remember he is very good technically and he is a great guy. He used to live very close to me.
"I could see the talent he has from training and playing the few games that I did with him. He has a presence.
"I would not say he is as broad as say Branislav Ivanovic but for a full-back he has the stature of a big guy."