Arteta was the main assistant. He has said exactly that himself and there have been various articles written by those that know these things, in the Athletic for example, that have talked about the fact that Guardiola entrusted Arteta to do much of the day-to-day training ground stuff in order to give him the time to work on strategies and problems outside of that.
City have also officially promoted one of Guardiola's long-time assistants, Rodolfo Borrell, to the position of Guardiola's right hand man. And you see this in practice when you watch City play now: he is in Arteta's previous position on the bench which is right next to Guardiola.
The reality is Arteta went to City as one of the assistants and quickly became much more important than that.
Also, I'm with Goon on the situation with the Gary O'Driscoll, the departing club doctor. I would imagine this move has been in the pipeline for a long time and the main motivation for his deciding to leave is to work with staff, coaches and players at what is currenty (sigh) the best team in the world as opposed to any friction with Arteta.
