The midfielder will have a medical, almost certainly on Saturday, and personal terms are also still to be agreed.
However, unless there are any hitches, Mata will make his debut in Tuesday's Barclays Premier League encounter with Cardiff at Old Trafford.
"Manchester United is pleased to announce it has reached agreement with Chelsea Football Club for the transfer of Juan Mata for a club-record fee," said United in a statement.
"The deal is subject to a medical and the agreement of personal terms.
"A further announcement will be made in due course."
Source : PA