RDM was ruthlessly axed instead meaning midfielder Mata, 24, faces Fulham tonight working for his third manager in less than 18 months at Stamford Bridge.
He said: "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."