Ozil, whose contract is set to expire next June, did not feature towards the end of last season and was not named in Arsenal's matchday squad for the first three games of the new campaign.
"The team is evolving and you can see the level they are achieving," Arteta said after Arsenal's 2-0 victory against Leicester City in the Carabao Cup on Wednesday.
"We want to evolve more and play better, so we need to maintain that. I try to select the players that are, in my opinion, in better condition.
"We are picking players we believe are the best for each game and as you can see, we keep changing.
"You can see ... that it is very difficult -- not just for Mesut but for some to make the squad. Every week, we try to pick the right players."
Ozil, 31, opened up about his issues at the club in August and said people have tried to "destroy" him at Arsenal for the past two years.
The playmaker also said that he intends to see out the final year of his contract before leaving the club next summer.