I would never say anyone is irreplaceable, a better question is can we replace him with anyone who is as effective.
I know there's a perception that he isn't effective but statistically, he is:
https://talksport.com/football/arsen...s-180131270735
Not this season. But it would be crazy to get rid of a player of that quality when a new manager is just coming in.
I don't believe any new manager would come in and think that Ozil is one of our big problems and we need to sell him.