I certainly don't think we can come to the conclusion that Haidara's a better defender yet, but one terrible game doesn't mean much either way at this point. My own view on these two is that Haidara looks promising (does show some very good promise defensively) but is as yet still an unknown quantity overall. Santon on the other hand has played plenty and IMO he just isn't a good defender, he can still improve since he's only 22, but right now he's weak defensively.
I'd love an explanation as to how he's poor defensively, as opposed to the oft-used argument of "he just isn't a good defender but I'm not really going to back that up with anything of substance".
He stops almost nothing coming in from the left. If an opposing player has possession there with Santon to beat, it's almost guaranteed he'll be able to play a ball into the box largely unopposed. I don't know how people miss this.