Once a player kicks out, referees usually leave themselves very little room. That is why the decision on Foden was always likely to stand.
That is the hard lesson for attacking players in derbies. However strong the grievance, the second act is the one officials see most clearly.
The debate did not stop with the dismissal because many felt Fernandes had played his part in creating the scene.
This is not new in elite football. Players test limits. Opponents look for reactions. The controversy grows when one player is sent off and the other appears to walk away untouched. That grey area is where pundit criticism usually lands.
The derby incident matters beyond one result because it taps into a wider concern: players, managers and supporters want the same threshold applied every week.
For Manchester City, the concern is how Foden responds if any suspension follows. For Manchester United, the spotlight will stay on Fernandes and how he manages those confrontations. The next time either side enters a high-pressure game, this episode will not be far from the conversation.