Valencia were reported to have rejected a bid from Chelsea for the Spanish international earlier this week but Mata has told the club of his desire to move to Stamford Bridge.
A deal for the 23-year-old is expected to be finalised on Saturday as lawyers negotiate the final details of the transfer.
Chelsea boss Andre Villas-Boas, who met Mata on Friday to discuss the role he has in mind for the player, admitted that the club could be making a new signing.
"We might just add a number in midfield," said Villas-Boas.
Valencia had been reluctant to sell Mata, despite their difficult financial situation.
Mata, who scored nine goals in forty three appearances for Valencia last season, rejected a move to Tottenham earlier this summer.