Di Matteo confirmed he had recommended to the Chelsea board that they keep Cole "as long as they can" before waxing lyrical about the 31-year-old, who controversially joined the club from Arsenal six years ago.
"He has become a legend - he is now Chelsea through and through," said Di Matteo.
"But I wouldn't say it's just happened recently. He's been here many years and has contributed a lot to this club."
Hailing Cole as an ideal example for Chelsea's new players to follow, he added: "He's a fantastic pro, which I'm not sure whether you know.
"Somebody that's the first one out in training, one of the first into the building with JT [John Terry], trains every day like a professional, reliable, somebody that you could go to war with.
"And he's a very quiet guy as well. He's very private at his own stuff.
"He's one of the best left-backs in the world."