Mourinho, the so-called 'Special One', dropped the double Chelsea Player of the Year during his time at Stamford Bridge before selling him to United in January 2014 for a then club record fee of 37 million pounds ($45.37 million).
Mata was adored by Chelsea fans but the Portuguese seemed to feel differently about him.
"We press very high up the pitch," Mourinho said at the time. "My attacking players have big distances to run between the initial pressure and the way they compact with the defensive players."
Most critics thought Mata would be swiftly shown the door when United's new manager took control but he is being given more responsibility by Mourinho this time and there have even been media reports of a new contract being offered.
The former Chelsea, Real Madrid, Inter Milan and Porto boss offered an explanation in September.
"First of all I didn't sell him," Mourinho told reporters. "My job is not to buy and sell, my job is to coach, to work on the pitch and to advise my board about transfer policy.
"Secondly it was Juan that asked to leave and when a player asks to leave I think always you have to think twice.
"The third point is that my idea for football with the squad and objectives we had at Chelsea is one thing, and the squad I have at Man United and the profile and what I try to do with this club is a completely different situation," said Mourinho.
"Juan at Chelsea in my project was one player and Juan in my project at Man United is another thing."
NUMBER 10 ROLE
Mata has made five league starts already under Mourinho this term, scoring two goals, and has finally occupied the preferred number 10 role that evaded him under previous managers David Moyes and Louis van Gaal.
Mourinho believes Mata has found his "natural habitat" at Old Trafford and the recent omission of club captain Wayne Rooney from the team has also helped the Spaniard's cause.
The former Chelsea and Valencia midfielder is a rare breed of footballer in the modern era.
Mata is working towards gaining a degree in business and marketing in a long distance learning arrangement with the University of Madrid and is a passionate photographer.
He is also a player who shows a genuine appreciation for his privileged position.
Take the video that emerged earlier this month, for example. Mata met a disabled fan and his father, holding up United's team bus in the process.
That is just one of many selfless gestures Mata has made during his career and he is involved in several charity projects in Britain and Spain.
Henrikh Mkhitaryan, who joined United from Borussia Dortmund for 26 million pounds ($31.88 million) in the close season, was expected to ease Mata out of the side but it is the Spaniard instead who has been given a chance by Mourinho to prove his worth.
($1 = 0.8156 pounds)
(Editing by Tony Jimenez)
