Chelsea and Manchester United have provisionally agreed a £40m fee for Juan Mata, though the champions are yet to make a formal bid and any deal to take the 25-year-old to Old Trafford remains dependent on him personally informing José Mourinho of his desire to move.

Negotiations between officials from the two clubs are at an advanced stage, with the transfer said to be "almost complete" by sources close to the discussions.

Despite the lack of an official offer from United for Mata, who has found first-team opportunities relatively limited at Stamford Bridge this season under Mourinho, the London club anticipate that situation changing soon as there is a little over a week of the current window remaining.

There is an instinctive reluctance at Stamford Bridge to lose their player of the season from each of his two full campaigns at the club, sentiment expressed publicly by Mourinho over recent weeks when the manager has been asked about Mata's immediate future. Yet the Spaniard has completed 90 minutes only three times this season, starting 11 league games, and, with the World Cup to come in the summer, his frustration on the sidelines has been acknowledged sympathetically.

Other clubs have expressed tentative interest in the forward, most notably Paris Saint-Germain, with Atlético Madrid going as far as to seek to take him on loan for six months with an option to make the move permanent in the summer. Yet the prospective deal that would be lodged by United would eclipse all-comers with Mata, who would be cup-tied in Europe this term, favourable to the idea, intent as he is to perform regularly for a prestigious club. David Moyes's side languish 14 points from the Premier League summit and six from the Champions League places, but the club retains its mystique.

Whereas in the past the Chelsea hierarchy have had the final say on outgoing transfers regardless of the manager's opinion, the decision now will rest with Mourinho. The Portuguese could yet block any move, though his stance is likely to be shaped by Mata's attitude. The Spaniard, who is contracted until 2016, would not naturally agitate for a transfer but, if he makes clear his desire to move in talks with the management, the club are expected to yield. Mata is due back at Cobham, with the rest of the Chelsea players, on Wednesday after two days off.

A similar policy was adopted over Kevin De Bruyne this month, with Chelsea's instinct to loan the player to Atlético only for Wolfsburg to bid £18m. The Belgian met key members of the club's hierarchy to make clear his desire to join the Bundesliga club, but it was only once he had confirmed as much in face to face talks with Mourinho that his departure was finally sanctioned. While Mata's would constitute a considerably more significant sale, the same principle will apply.

United, who discovered on Tuesday that captain Nemanja Vidic will be banned for three games after losing his appeal against the red card he was shown in the recent 3-1 defeat at Chelsea, remain resolute that Wayne Rooney, a player Chelsea have coveted since last summer, will not move to Stamford Bridge as part of any prospective deal for Mata. Indeed, their pursuit of the player represents something of a change in policy given their priority had previously been a left-back and a more classic central midfielder rather than a creative player whose desire is to play as a No10, where Rooney, Shinji Kagawa and Adnan Januzaj can also operate capably.

Yet the interest reflects an urgency to make a marquee signing – it would eclipse the £30.7m spent on Dimitar Berbatov from Tottenham Hotspur in 2008 – and signal United's intent still to secure Champions League qualification this term after recent toils. His arrival would lay down a marker as the club seeks to rebuild its squad with players of similar quality.

Chelsea had originally hoped to review Mata's situation in the summer, conscious as they are that they will struggle to recruit a player of comparable calibre in the remainder of this window and with challenges ahead on three fronts. Indeed, Juan Mata Sr, who acts as the player's agent, had hoped to broker a deal to Barcelona at the end of the campaign, only for United's interest to emerge and prove enticing.