The 24-year-old midfielder revealed his shock at the Italian coach's departure following the Champions League defeat by Juventus, but thinks he will be managing again soon
The former boss won a Champions League and FA Cup double after less than a season in charge, but was dumped by club owner Roman Abramovich on November 21 as his side looked to be heading out of Europe.
But Mata - who has now played under three managers at Stamford Bridge in less than two seasons - feels the 42-year-old will be back in the game before too long.
"Roberto [Di Matteo] won two trophies in just nine months which is very difficult to do, so you have to recognise the job he did," he told reporters. "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."
Mata recalled the celebrations after the unlikely double which followed the sacking of Andre Villas-Boas, saying Di Matteo gave the club memories which the fans will be forever grateful for.
"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."
And while Di Matteo may currently be out of a job, the Spain midfielder believes it is only a matter of time before another club picks up a young manager with such impressive recent successes on his CV.
"He's a very good coach and a legend at Chelsea, people have such a lot of affection for him," Mata continued.
"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."
Chelsea started the 2012-13 season brightly under Di Matteo, winning seven of their eight opening Premier League games.
However, two points from a possible 12 and a likely exit from the Champions League after a disastrous 3-0 defeat by Juventus sealed the manager's fate, with Rafa Benitez now the interim boss until the end of the season.