The 23-year-old only arrived at Stamford Bridge last month in a deal worth £23.5million after spending four years at the Spanish club.
And any doubts over what reception he'll receive from the fans who used to idolise him were quickly allayed when he was mobbed at the airport when the Premier League side touched down in Spain.
But when the game kicks-off tonight, Mata knows that will change, and the men he calls friends will become enemies, if only for 90 minutes.
"Pablo [Hernandez] is a very good friend of mine but, after the kick-off, that's where friendship will end," Mata said in the pre-match press conference.
"After the game, it will be a different story again.
"It's going to be strange to be in a different changing room. But that will make the game special.
"I know about the atmosphere at the Mestalla when Valencia play a Champions League game. I know how special the game is going to be and I'm really happy to be here.
"Wednesday I'll find out what playing against my former team means.
"Before the game and after the game, I'm going to say hello to my friends and everybody here, but during the game I'll be focused on helping Chelsea win."