Just going to completely ignore the words "one of" before the bit you bolded?
Delph is a completely different scenario. He certainly isn't first choice, he doesn't play when others are fit and is different further still seeing as he's homegrown. Both of Otamendi and Mangala wouldn't play in our best side, only one currently would but we're seemingly in for another defender anyway to replace him and it's been 50/50 with regards to Kolarov being first choice.
Additionally, i'd add, the defence can still for now hide behind us not really being able to judge them under Pellegrini's interpretation of defending. Navas on the other hand has no such excuse, His record speaks for itself, I think i'm right in saying Navas played the most minutes last season for us and was very clearly first choice on the right. the ones you mentioned either aren't 1st choice or won't be before the start of the season so yes he should be ONE OF the first to be replaced. Replaced doesn't necessarily mean sell either.
If Navas played for somebody else and was linked with a move here we'd all think Txiki had lost the plot, he's simply not good enough to be a starter for us regardless of how much effort the guy puts in. Anybody that values effort enough to overlook a lack of quality needs to get with the times and realise we're a club that competes and wins trophies nowadays and effort alone isn't enough.