Di Matteo was one of the first people Mancini looked up when he was considering whether he should accept the offer to replace Mark Hughes last December.
"Roberto played here for many years at Chelsea and also knows Italian football very well," confirmed Mancini.
"He explained a few things to me.
"He is a good guy and a good manager, who did a good job last year.
"It will be difficult for us tomorrow because he knows everything about us."