"Valencia Football Club have reached an agreement in principle with Chelsea FC for the transfer of the player Juan Manuel Mata," a club statement read.
"The agreement is subject to the relevant medical tests."
The 23-year-old, also a target for Chelsea's faltering rivals Arsenal, has 11 Spain caps and has been mainstay of the Valencia side since 2008 with his speed and goals marking him out as one of Europe's most exciting wide-men.
Chelsea have signed Belgium striker Romelu Lukaku and Spanish midfielder Oriol Romeu in the transfer window as new coach Andre Villas-Boas looks to shine in the Champions League and snatch back the Premier League title from Manchester United.
Fans often bemoaned Chelsea's lack of width last season under Carlo Ancelotti.
Valencia, third in La Liga last term, were demanding around €30 million for Mata, according to media reports.
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