"We are convinced that he is going to do well. Manchester United pay a record fee because they know he is a player that can be important for them, it is not a gamble," Mourinho told a news conference on Friday, with the deal still to be formally completed.

"We are convinced he is going to do well and we wish that. When we play him we will try and win and he will be the same, but everyone in this club wishes him well."

United have reportedly made a club record bid of �37 million ($61m, 44.6m euro) for Mata.

The midfielder was Chelsea's player of the year in his two seasons since joining from Valencia but has struggled to cope with Mourinho's demands for a greater work ethic since the Portuguese returned to Stamford Bridge with Eden Hazard, Oscar and Willian favoured in the three attacking positions behind a lone centre forward.

Mata has completed just one of the last five games he started and last featured on New Year's Day at Southampton, where he reacted angrily to his substitution early in the second half.

Chelsea are currently third in the Premier League table, two points adrift of leaders Arsenal with reigning champions United 14 points off top spot.

But the imminent arrival of Mata will be a huge boost to a United who, already knocked out of the FA and League Cups, have been struggling in their first season since David Moyes replaced the retired Alex Ferguson as manager at Old Trafford.

Mata's departure from Chelsea seemed all the more likely when Chelsea revealed on Thursday revealed they are set to win the race to sign Egypt winger Mohamed Salah from Swiss club Basel.

Mourinho has made Salah his top target to replace Spain Mata and he is on the verge of persuading the 21-year-old to move to London despite strong interest from Liverpool.