Mata, 23, has been strongly linked with moves to Arsenal and Liverpool, while Manchester City and Tottenham are also reportedly tracking his availability.
Mata, who has been capped 11 times by the senior Spain side but led their Under-21s to European Championship glory earlier this summer, claims he is happy at Valencia and focused on winning trophies at the Mestalla.
But Emery admits the club may have to sell if a large enough bid is put on the table.
"Valencia has to be prepared for a footballer having to leave, either because is he is not wanted or because some big club wants to sign him," Emery told Cadena Copa.
"We want him to stay and Mata has also expressed his wish to remain at Valencia, but we know that if a big-money offer comes in from any club and it's good for both the player and the club, it could lead to his exit."