Matic, 30, has been an ever-present in Uniteds starting XI since his move from Chelsea in the summer of 2017 but the midfielder has come under scrutiny after a string of below-par performances this season.
While Matic had a strong start to his United career, the Serbians form has noticeably dipped and his display in Sundays comprehensive defeat to Manchester City – alongside Ander Herrera and Marouane Fellaini – saw him face yet more criticism.
He was partly to blame for Ilkay Gundogans goal, Citys third, at the Etihad after some lacklustre marking of the former Borussia Dortmund man.
According to The Times, Mourinhos refusal to drop Matic is beginning to cause tension inside Uniteds dressing room.
Its claimed staff and players have been left bemused by Mourinhos unwavering support of Matic despite the midfielders lack of mobility causing serious concern.
Matic made 35 starts in the Premier League for the Red Devils last term and has started nine of the sides 12 opening matches this season.
Mourinho still refused to point the finger at Matic after the Manchester derby and instead claimed that his game plan was ruined because he was forced to start Fellaini.
Asked if United missed Paul Pogba, Mourinho replied: Its not me after a defeat to be speaking about a player who was not here.
Of course, we missed him. One of the things that broke our plan a lot was the fact Fellaini had to start the game.
Because Fellaini is not in conditions to play 90 minutes. He was phenomenal in his effort but he was not in the condition to do it and normally, I bring Fellaini to this game in the latest part of the game.
Imagine this game with Fellaini coming on at 2-1 fresh with 25 minutes to go.
Thats an important thing for us.