Former Liverpool manager Rafael Benitez is set to be named as Di Matteo's replacement on a short-term deal, as revealed by Goal.com on Wednesday morning.
But as the Chelsea players bade farewell to the manager who led them to a maiden Champions League triumph six months ago, Mata took to Facebook to pay tribute to his outgoing boss.
"It's been a difficult day," said Mata. "Roberto Di Matteo is no longer our manager.
"I would like to thank him for all this time with us and wish him the best of luck for the future.
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"He will always be remembered. Good luck Robbie!"
Ruud Gullit first brought Di Matteo to Chelsea in 1996, when the Italian joined the club from Lazio for what was then a club-record fee of £4.9 million.
Injury forced Di Matteo retire in February 2002, but he returned to Stamford Bridge in June 2011, when he was named as assistant manager to Andre Villas-Boas.
When Villas-Boas was sacked in March 2012, Di Matteo was appointed caretaker manager and won the FA Cup and Champions League, before being offered a two-year deal to take the job permanently.
However, after the Blues' defeat to Juventus last night extended their current run to just one win in eight games, the Chelsea hierachy decided it was time for a change.