Just his stature alone makes him the best option for the starting berth. With KWP there, we look flimsy and light weight. Aurier is stronger, harder in the tackle, more experienced and a far more solid bet. The same goes for Foyth who is certainly anything BUT solid. I think any question marks around Aurier (for me there aren't really any) - a player who was a starter for PSG - then those same questions marks hang above Foyth's head with bells on.
People level ill-discipline at Aurier, but the sheer recklessness Foyth has displayed - including the awful red card at Bournemouth last season - far exceeds anything being levelled at Serge. When you factor in that Aurier is far superior in an attacking sense too, as underlined by his crosses for goals last season against Dortmund, the few goals he has picked up himself, and the assists yesterday, then it is not even close.
IMO, unpopular as it may be, I don't think Poch is always the best and getting the most from his players. We're seeing it (again IMO) with Moura and its absolutely been the case with Serge. He's a good player, better than Trippier, but I believe he has come in for some of the weird stuff from Poch. Now is absolutely the time to get him in the team, first choice, week in & week out. He is waaaaaaay more solid than anything else we have and helps us look like more of a proper balanced team.