Hi buddy. I never sub a GK, even if he gets injured. Makes no difference from what I've seen. With defenders, if one is having a nightmare, you will often see it's because the opposition attacking player he's up against is having a blinder. First I will try another defender swapping position with him but if I think he is dragging my team down, I'll sub him. But you're right, the sub usually performs as badly because it seems that particular position is dominated by the opposing attacker.