• Deal of around £40m dwarfs £30.75m paid for Dimitar Berbatov • 25-year-old could make his first appearance on Tuesday

Manchester United have signed a transfer agreement with Chelsea to buy Juan Mata for a club record fee of around £40m, surpassing the £30.75m paid for Dimitar Berbatov in 2008. The playmaker's arrival hands a massive boost to David Moyes in what is proving a difficult first season in charge of United.

Moyes has also received positive vibes from Wayne Rooney who, following the uncertainty over his future, is minded to sign a new deal if the terms are suitable, with the striker wanting an increase on his £250,000-a-week contract that expires at the end of next season.

Discussions between Rooney's advisor, Paul Stretford, and Ed Woodward, United's executive vice-chairman, are continuing, though sources close to the talks are clear that an agreement remains in the balance. The 28-year-old would be turning his back on Chelsea, Arsenal and Real Madrid should a new contract be thrashed out.

Mata will have a medical in Manchester and then discuss personal terms. Both are expected to be a formality and the 25-year-old could be announced as a United player as early as Saturday evening and be available to make his debut against Cardiff City at Old Trafford on Tuesday. Any new addition has to be registered by noon on Monday.

A statement from the champions said: "Manchester United is pleased to announce it has reached an 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."

The purchase of Mata represents a coup for United given that he was Chelsea's player of the year for the past two seasons and acquiring top-class players in the January window is particularly difficult.

The signing of the transfer agreement – the key piece of paperwork in terms of Mata's destiny – had been delayed as Chelsea wanted to secure the signing of Mohamed Salah from Basel.

José Mourinho said: "Salah is a non-EC player so he cannot come to England for his medical, so we have to meet him for the medical, probably in Switzerland. After that we can do the paperwork for him to come."

The Chelsea manager indicated that Mata would become a United player. "Things are going in a good direction. We have allowed Juan to travel and have the medical. The player deserves respect and to play and because the offer is right, we allowed him to negotiate. It's what he wants. It's what he asked us to accept."

He added: "Chelsea is proud to act this way, in an open way, in a human way. It's good for Juan, it's good for the Premier League and the offer in financial terms is the right offer.

"The offer, compared with what the club paid two years ago [when Mata signed from Valencia], was fantastic. If Man United pays their record for a player, it's because they know exactly what they want.

"They are convinced he's going to be important for them and we hope that he does very well for them. No fears at all. We are convinced he's going to do well."