Manager David Moyes has already confirmed Mata will be in his squad for Tuesday’s Premier League home game against struggling Cardiff.
Moyes said: “I know our fans have admired Juan for some time now – he has always played well against us – and I know they will be delighted to see him in the red of United; starting on Tuesday.”
The sum spent on Mata smashes United’s previous record outlay – the £30.75m they paid Tottenham for Dimitar Berbatov in 2008.
It underlines the desperation at Old Trafford to prove they are determined to shove United back to the top end of the Premier League after this season’s disastrous campaign.
United have slumped to seventh in the table and were eliminated from the League Cup semi-finals by 19th-placed Sunderland on Wednesday.
In addition, they have also been knocked out of the FA Cup, meaning an unlikely Champions League triumph is now their only hope of silverware, in addition to currently being their best route back into the competition next term.
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Despite the strife, Mata had no hesitation in jumping at the chance to rebuild his career, which has stalled following Jose Mourinho’s arrival at Stamford Bridge, even though he was crowned Chelsea’s player of the year in the previous two seasons.
He said: “I am thrilled to be joining United. I have enjoyed some very happy years at Chelsea but the time has come for a new challenge.
“United is the perfect place for me and I am excited at the chance I have to be part of the next phase in the club’s history.
“Chelsea is a top club and I have many friends there but you cannot turn down the chance to join Manchester United.
“I look forward to helping the manager and the team be successful in the coming years.”
Mata is ineligible for the Champions League, meaning his task for the remainder of this season will be to inspire a team whose recent performances have been appalling by their own high standards.
Moyes has clearly noted a lack of invention in his squad, which has only been accentuated by the recent absences of Robin van Persie and Wayne Rooney.
Having been a regular for Spain – one of the major reasons behind Mata’s desire to move was to cement a place in his country’s World Cup squad next summer – he has the experience and ability to have the kind of instant impact Moyes craves.
“Juan is one of the finest playmakers in the game today and it’s a real pleasure to have secured his signature,” said Moyes.
“He’s been instrumental in Chelsea’s recent success, with a notable 28 assists and 32 goals in his last two Premier League seasons.
“I have been impressed with the excitement he has shown to be part of the new Manchester United.”
As for Rooney, the 28-year-old has 18 months left on his current deal but moves have been made to look at extending that with an improved offer.
Rooney’s agent Paul Stretford was pictured attending the club’s Carrington training ground this week.
The England international is reportedly still a target for Chelsea manager Jose Mourinho, whose decision to allow midfielder Juan Mata to move to Old Trafford may actually improve the Red Devils’ chances of securing Rooney on a longer-term contract.
Prior to Rooney signing his previous deal, the player expressed his unhappiness with the direction the club appeared to be taking and the lack of top-class talent they were recruiting.