I don't think it is poor coaching but poor team building. Grier consistently puts together that can be competitive if absolutely nobody gets injured, overpaying the top 20 stars and completely ignoring the rest of the roster. Championships are not won by having the best starters but by having backups that can step in.
I think having backups that can step in comes from having good coaching. Starters usually have the most natural talent, competent backups come from working hard and getting good coaching.