Mata also revealed the influence of Fernando Torres in his decision.
"I want the Premier League title and this is possible at Chelsea.
"Fernando Torres helped influence my decision but speaking directly with Villas-Boas was the decisive factor.
"He explained how his style will be based on 4-3-3 and how important I will be to his plans.
"Frank Lampard is a monster of a footballer to play alongside and the prospect of that is incredible."