There is absolutely no reason not to play Nkounkou.
If Delph or Iwobi or whoever manage to have a good game there what benefit does it give us other than a short term gain? Neither are the answer there (not even have a going at them, I mean it's obvious, Iwobi did OK at Right wing back last week, asking him to do the same on his weaker side is daft, Delph is nowhere near energetic enough to be a modern wing back or full back in 2020.).
Every minute Nkounkou gets on the pitch right now benefits both him and us as at very worst he should be our back up left back for the next few years, at best he plays well and gives the manager a headache when Digne returns. I mean whats the worst that happens if he has a mare? The crowd get on his back? There is no crowd.
I hope he was injured today or something because if not It will annoy me the more I think about it.