I've seen people defend Keane last 2 games saying "he wasn't our worst defender".
The issue is that his mere presence changes the way others play. Even at bottom of the barrel level football, I've played with players who are far worse than me and I've had to work twice as hard to compensate, but mostly I've played with players far better than me get frustrated when I can't do what they expect, ans that's just random blokes hiring a 5g pitch for a game of casual footie. Tarks now has to work twice as hard with him next to him and because of that is more likely to make mistakes himself.