Manchester United had to change their game strategy in the next match, Manchester United players were too open because they were obsessed with immediately scoring goals from the counter-attacks they built. Facing a team like Manchester City, they have to play carefully and close every empty room, Manchester United players don't do it so they have to accept the fact that they conceded 6 goals.
Before the big defeat in derby match, Manchester United were considered as doing well enough. They improved at least in results after first few terrible matches but the international break and the derby bring them back to reality. They are no where closely to level of top clubs in Premier League at the moment.
In addition, Manchester City are simply too strong than most of clubs in Premier League, not only Manchester United. Fortunately, depends on how you see this, this defeat reveals weakness of United defense and from that Ten Hag can make plans to improve it. I am sure that United won't be able to compete fairly with City in this season but they can improve and compete with other clubs. At least competing for a position in top four and a chance to play in Champions League next season.
That target is important for them to attract star players in next summer that will be helpful for their long term development plans.