#2 Swallow pride

Mourinho needs to put his pride aside and give full priority to the team. In the past, he has put good players in the cold simply because they disagreed with him. Mourinho would rather play under performing players than give a chance to players he has clashed heads with.
Eric Bailly had just one or two bad games this season and Mourinho had a disagreement with him, after which Bailly was completely frozen out.
Bailly made a mistake against Newcastle and he was whisked away after only 20 minutes. Nobody suffered the same fate in Mourinho's back three of two holding midfielders even when they went 2-0 down against Southampton inside 20 minutes.
If fit, Bailly should be given a chance to prove himself as he would strengthen the defense considerably. Also, Antonio Valencia should be reinstated as a starter to steady the ship.
#1 Identify the right formation

Mourinho should set a 4-4-2 formation against Arsenal because that gives the team more balance. At the heart of midfield, he would have Nemanja Matic and Ander Herrera as cover for Paul Pogba.
Juan Mata is a creative force who can cause serious damage as a number 10. Jesse Lingard is also a suitable choice in that position.
Then, he should deploy the clinical Anthony Martial and Marcus Rashford, who was impressive against Southampton. This means dropping Romelu Lukaku altogether.
Rashford, Lingard and Martial will offer United the much needed mobility up front to cause Arsenal some problems. Marouane Fellaini, meanwhile, can be brought on as a substitute in the second half to destabilize Arsenal.