Mata arrives for United medical
The Old Trafford club posted a photo on their Twitter feed which showed Mata being greeted by United boss David Moyes after arriving by helicopter at the club's training ground.
United and Chelsea have already reached agreement over the transfer but the Spain star still has to agree personal terms and pass his medical.
United had confirmed on their Twitter feed on Friday evening: "MUFC has reached agreement with Chelsea for the transfer of Juan Mata for a club record fee.
"The deal is subject to a medical and the agreement of personal terms. A further announcement will be made in due course."
And Chelsea confirmed: "Chelsea Football Club has reached an agreement with Manchester United for the transfer of Juan Mata. The move is subject to a medical and the agreement of personal terms."
United will reportedly hand over £37m for Mata, far exceeding their previous club record of £30.75m they spent on Dimitar Berbatov in 2008.
The fee also represents a hefty profit for Chelsea on a player they signed for £23.5m from Valencia in 2011.
Speaking at his Friday afternoon press conference, Chelsea boss Jose Mourinho wished Mata well.
He said: "He's a fantastic player and despite not playing a lot for us in the last period could be a very important player for us. But Juan had this fantastic opportunity for him.
"It's what he wants. It's what he asked us to accept. Chelsea Football Club is proud to act this way, in an open way, in a human way."
Mata was named Chelsea's player of the year in each of the past two seasons but has been reduced to a bit-part role since Mourinho returned to Stamford Bridge.
"The offer we've received we feel is the right offer for the quality of the player," Mourinho said. "It's a good move for Juan, it's a good move for the Premier League and the offer is the right offer. He was fantastic for Chelsea, and Chelsea were fantastic for him."
Responding to criticism of the sale from some supporters, he said: "If the fans are sad that Juan is leaving, I will say that we are also sad. No one is jumping around the place."