The Spain midfielder was jettisoned by Mourinho at Chelsea in January 2014 despite being a darling of the crowd, having won the Player of the Year award in his two previous seasons at the Stamford Bridge club.
Mata is an astute attacking player who is always liable to chip in with an important goal, as he showed when scoring United's equaliser in their 2-1 FA Cup final win over Crystal Palace at Wembley on Saturday.
He does not fit the bill, however, for a prototype Mourinho midfielder. He lacks defensive attributes because he has no pace, is weak in the air, cannot tackle and is heavily reliant on his left foot.
“It is one thing to play with Ramires and Oscar closing down opponents on each side and Mata as a number 10 behind a striker with his clever assists, clever passes and fantastic actions because he has great talent,” said Mourinho in September 2013.
“But it is another thing to adapt to the way we want to play.”
No matter how skilful they might be, Mourinho demands all his midfield players track back when the team lose possession to form a solid defensive shield.
Within months of selling Mata to United, the Portuguese replaced him at Chelsea with Cesc Fabregas.
The Londoners won the Premier League at a canter in Fabregas' first season at Stamford Bridge but in the latter part of the campaign Mourinho was reluctant to employ the Spaniard in a holding role alongside Nemanja Matic, thrusting him further forward where his defensive shortcomings were less noticeable.
Mourinho would have liked to sign the more combative Paul Pogba from Juventus after winning the league but Chelsea's advances fell on deaf ears, and the club endured a woeful start to the season that eventually led to the manager's sacking in December 2015.
A possible scenario, now that Mourinho is set to hold the reins at United, would see France international Pogba returning to his first club as one of the linchpins of a new-look team.
Daley Blind is another United player whose role will come under scrutiny.
It seems inconceivable Mourinho would entertain the thought of deploying Blind in the same central defensive role he occupied under Van Gaal.
Mourinho prefers hard-as-granite centre halves like John Terry of Chelsea, Real Madrid's Pepe and former Stamford Bridge stalwart Ricardo Carvalho, players that rely on their defensive attributes rather than eye-catching skill at the back.
Blind, knowing perhaps that it might be tricky to win Mourinho over, wanted Van Gaal to stay at United.
“I always worked well with him and would have been keen that the co-operation lasted longer,” said the Dutchman after breaking off from training with the national team squad as they prepared for next month's European Championship in France.
“Even though there was so much being written about him losing his job over the last months, he always looked to protect and shelter us.”
Bastian Schweinsteiger, Memphis Depay, Ashley Young and Adnan Januzaj are other United players who may be understandably apprehensive about Mourinho's impending arrival.
The Old Trafford fans have never really taken Marouane Fellaini to their hearts but the Belgium international is powerful in the air and combative on the ground and may be someone the Portuguese can work with.
As for another possible marquee signing, media reports have persistently linked Sweden striker Zlatan Ibrahimovic with a move to United from Paris St Germain.
Ibrahimovic teamed up with Mourinho at Inter Milan and has always spoken highly of the Portuguese.
“Mourinho is the disciplinarian,” said the Swede. “Everything with him is a mind game, he likes to manipulate.
“Such tricks were new for me, all the time doing one thing to get another thing, all the time triggering me. I like these games and it worked for me.”
Original source: Mata unlikely to be a Special Juan for Mourinho