You could ask 'what if' about any incident in any game and spend the rest of your time creating fantasy matches in your head, which is all this is.
I can see your logic. What might have been if he has missed against Man U at the same end of the ground ?
However he has been one of the shining lights this season
Players don't become exempt from being substituted just because they've scored a goal. Especially one that would have been scored an hour before the sub was made.