The 23-year-old, who joined the Blues in a £23.5million deal barely a month ago, was treated to a hero's welcome after touching down in Spain.
But Mata knows he will be the enemy on Wednesday when he walks out at the Mestalla in Chelsea's Champions League Group E game against his former club.
Good friend and Valencia winger Pablo Hernandez has already said as much and those sentiments where echoed by Mata on Tuesday night.
"Pablo is a very good friend of mine but, after the kick-off, that's where friendship will end," Mata said.
"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.
"Tomorrow [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."
Mata, who made his Champions League debut in Valencia's draw at Chelsea in 2007, revealed Chelsea boss Andre Villas-Boas had not sought to exploit his inside knowledge of the club where he spent four years.
"The coach knows the Valencia team and players," he said.
"He knows they have played really good games this season and he hasn't asked for any report at all."
Chelsea and Valencia are favourites to progress from Group E.
Mata said: "I wish that both teams go through, but obviously tomorrow [Wednesday] night I want to win the game.
"I know that Valencia will be a great threat tomorrow [Wednesday], especially after the first game, where they drew in Genk.
"Tomorrow's [Wednesday] game is especially dangerous. In the Champions League, you have to get three points and I believe that Valencia will be trying that tomorrow [Wednesday]."