Ozil was not selected in Mikel Arteta's groups for either domestic or continental action for the remainder of 2020.
The former Germany international, who last played against West Ham in March and was on the bench just twice after the Premier League resumed in June, said his loyalty to the Gunners "has not been reciprocated".
Ozil is continuing to train with Arteta's side, though, and Mustafi - another member of Germany's 2014 World Cup-winning squad - says the midfielder is "doing the best he can".
This is a difficult message to write to the @Arsenal fans that I’ve played for over the past few years. I’m really deeply disappointed by the fact that I have not been registered for the @premierleague season for the time being. Upon signing my new contract in 2018, I pledged my loyalty and allegiance to the club that I love, Arsenal, and it saddens me that this has not been reciprocated. As I have just found out, loyalty is hard to come by nowadays. I've always tried to remain positive from week to week that there's maybe a chance to get back in the squad soon again. That's why I kept silent so far. Before the Coronavirus break I was really happy with the development under our new coach Mikel Arteta - we've been on a positive way and I would say my performances were on a really good level. But then things changed, again, and I was not longer allowed to play football for Arsenal. What else can I say? London still feels like home, I still have many good friends in this team, and I still feel a strong connection with the fans of this club. No matter what, I will keep fighting for my chance and not let my 8th season at Arsenal end like this. 🙏🏼🙏🏼 I can promise you that this hard decision won't change anything in my mindset - I will continue to train as best as I can and wherever possible use my voice against inhumanity and for justice. #M1Ö #YaGunnersYa
"I just know that Mesut, for me, is not a team-mate - he is more than a team-mate, a friend," Mustafi said.
"I respect him. He always has respected me and everyone else, as far as I have seen, so he is always going to have my respect and I am always going to be there for him if he needs something to motivate him and push him.
"But at the moment, every time I see him at training, he's doing the best he can. He tries to train well, to be there."
Asked if he thought Ozil had expected to be left out, Mustafi replied: "I can't look into his head. I can only see what he's doing at the training ground and what he's doing on the pitch.
"I felt that he was always trying his best. At the end of the day, no-one expects or wants to be left out.
"He's a friend. I'm always going to respect him, I am always going to be there if he needs someone to motivate him, to push him to keep going.
"But he is so long in this business and has played so many games that he doesn't really need to be motivated. I see him very motivated.
"Of course, when you're not in the squad, you're going to be disappointed - even him.
"The last thing you want to see is someone left out and not being disappointed. This would be the wrong thing. Of course he was disappointed.
"On the other side, he's experienced enough to deal with the situation. For me, the main thing is how I see him on the training pitch and there's nothing I can say negative about him."
Ozil has the sympathy of former Arsenal colleague Jack Wilshere, who was similarly restricted to a fringe role at West Ham before his contract was terminated.
"I was in that position," Wilshere told Sky Sports. "As a footballer, you want to be given the opportunity to compete and fight for your place.
"He's one of the best players I've played with. All I can do is comment on him as a footballer and I know what he can give, so it does make it difficult to understand.
"But I'm not in the club, I don't know what's happened. I'm sure for a lot of teams in England and Europe, Mesut will be the first name on the team sheet.
"But that's why Arteta's the manager. He makes the decisions and that's down to him."