The Red Devils finally completed the signing of the Serbia international from Premier League champions Chelsea on Monday evening.
Matic will reunite with Mourinho at Old Trafford after the Manchester United boss signed Matic in a £21m deal from Benfica in January 2014.
The holding midfielder won the Premier League title and the EFL Cup crown with Mourinho before the Portuguese coach was sacked as Chelsea boss in 2015.
Matic was facing an uncertain future at the defending champions after Mourinho’s permanent successor Conte signed Tiemoue Bakayoko in a £39m deal from AS Monaco last month.
Speaking to Manchester United’s website about his decision to sign Matic, Mourinho discussed the attributes that make Matic an impressive summer signing.
“Nemanja is a Manchester United player and a Jose Mourinho player,” Mourinho said.
“He represents everything we want in a footballer; loyalty, consistency, ambition, team player.
“I would like to thank him for his desire to join us because without that, it would be impossible to have him here.
“I am sure our players and supporters will love him. A big welcome to our new number 31.”
Manchester United started their summer spending with the £30.7m signing of Sweden international Victor Lindelof from Portuguese champions Benfica.
The Red Devils beat Chelsea to Romelu Lukaku’s signature after the Europa League winners agreed a £75m fee with Everton for the Belgian striker.
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