Matic was withdrawn early in the second-half in the Red Devils' 4-2 win over Watford on Tuesday, 28 November, and was replaced by Ander Herrera. The Portuguese tactician confirmed his key player has suffered an "important" injury.
"For Nemanja Matic to tell me he wants to be replaced, he does not do that if it's nothing important. It is muscular," Mourinho said in his post-match press conference.
Earlier reports also suggested that the Serbian international was doubtful for the clash against Arsene Wenger's side. The Mirror now claims Matic is likely to be dropped from the squad to face Arsenal.
The 29-year-old moved to Old Trafford from Chelsea before the start of the season, and has impressed since leaving Stamford Bridge for United, starting every Premier League game this campaign. But it seems like his record of playing in every Premier League game this season could come to an end.
The report claims Matic is battling to be fit for United's trip to the Emirates after undergoing tests on the injury. However, the game against Arsenal may come too early for the ex-Blues midfielder.
Mourinho wants Matic to be fit in time to face Manchester City in the crucial derby clash at Old Trafford on 11 December.
In other Manchester United injury news, Marouane Fellaini did not travel with the United squad to Vicarage Road to face Watford on Tuesday. The Belgium international suffered a knee injury and remains a doubt to face Arsenal too. Meanwhile, Michael Carrick is recovering from a heart condition and Saturday's match may come too early for him as well.
Mourinho is likely to start Herrera alongside Paul Pogba in midfield against Arsenal.