The manager remains unsure on whether Matic will be able to make his 15th consecutive league start for United as they continue to chase league leaders Manchester City, who are eight points clear at the top.
"I don't know if he can play," Mourinho told a news conference on Friday. "He's travelling with the team, he's going to try and hopefully he can, but I cannot guarantee he is ready."
Mourinho has yet to win an away game against a fellow top six rival since taking charge at Old Trafford, with United's seven games at Chelsea, Arsenal, City, Liverpool and Tottenham Hotspur yielding three points and one goal.
When asked if Saturday's fixture was a must win to keep the title race alive, he replied: "I tell my players just to think about the game. It doesn't matter what happened before or what happens afterwards.
"We don't think about how many points ahead of them we are or how many points ahead the leaders are.
"It will be tough enough against Arsenal so we will focus totally on this game."
Arsene Wenger's side, fourth in the league and four points behind second-placed United, have won all their seven league games at home this season, scoring 20 goals and conceding four.
"When we have the ball, we are going to attack with 11 players because even the goalkeeper needs to know what to do when we have the ball," Mourinho added. "When Arsenal has the ball, we are going to defend with 11 players."
Mourinho confirmed defenders Eric Bailly and Phil Jones are not available for selection after missing their last three league matches through injury.
(Reporting by Hardik Vyas in Bengaluru; Editing by Christian Radnedge)
