The 23-year-old has been linked with several Premier League sides this summer, including Arsenal, Liverpool and Tottenham Hotspur, but Valencia President Manuel Llorente had been insistent that the player was not for sale.
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But now the club's manager has hinted Mata, a member of Spain's victorious side from this summer's European Under-21 Championships, could be sold if the deal is right for the Mestalla side.
"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 a club and it's good for both the player and the club, it could lead to his exit."
Mata has played 172 matches for Valencia since arriving from Real Madrid in 2007, scoring 46 goals.
He was also a member of the Spain squad which won last summer's World Cup, though he only made one substitute appearance during the tournament.
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