Di Matteo led the Blues to historic UEFA Champions League success last season, as well as FA Cup glory, but was shown the door last week after a drab run of results.
Mata, who was signed by Andre Villas-Boas at Chelsea, has now worked under three managers in less than 18 months at the Premier League club following Rafael Benitez's appointment last week.
The 24-year-old, who will host Fulham with the Blues in Wednesday night's West London derby, retains great respect for Di Matteo.
He told The Sun: "Roberto won two trophies in just nine months which is very difficult to do, so you have to recognise the job he did.
"But he has been sacked - that's how hard and unjust football is.
"Just six months ago we were celebrating having won the Champions League.
"I'll always remember the joy of our fans after we won the FA Cup and Champions League. Seeing thousands of people packing the streets to celebrate the trophies with the team is something I'll never forget."
Mata believes it will not be long until Di Matteo is back in another management role, adding: "He's a very good coach and a legend at Chelsea, people have such a lot of affection for him.
"Yes, there were hard and difficult moments, as there can be at any club, but they were more than made up for.
"And I'm sure he'll soon have the opportunity to coach again."