I would undermine your comment by pointing out that yes Dier has improved under Jose but so Has Toby who has been excellent in many of our recent games Aurier has also improved considerably. On top of that our defence has been much better organised and all players have increased their defensive awareness, Kane has made a few penalty box clearances as well. I don't buy into our defence being as poor as some describe, far from it, they just have too much to do. No defence will withstand without some problems for the best part of 80 minutes of a 90 minute game or even 60 or 70 minutes which is why I would like to see us using our considerable attacking skills much more than we are.
Joes is successful because he is very strong minded but sometimes you have to doubt yourself a little to move into somewhere new.
I agree with you that Aurier has improved considerably though he does always have a brainfart in his locker which always leaves me worried when he plays, though unfortunately Doherty has been even worse when he's played so right now you wouldn't drop Aurier.
The CBs though, I just don't feel safe with any of the three senior ones at the moment. I feel between them, they lack concentration and real defensive steel. For me, if we compare any combination of the three to that of the prime Jan and Toby partnership under Poch, or the King and Woodgate one under Harry (which we only got for one whole season due to injuries
Another way I'd judge if the quality is good enough is would any of our three CBs start for any of our current top four/ top six rivals at the moment?
None would get in at City or Pool, don't think they'd get in at Utd, wouldn't get in at Leicester nor Everton either in my opinion. If we add Chelsea or Arsenal to the mix as they're usual top six teams, we'd probably get one or two in at Arsenal alongside Gabriel. Chelsea's CBs are probably on a par I'd say, can make a case for both sets of CBs.
If we could ideally bring in our own "van Dijk" like Pool did, that may work wonders and improve whoever plays alongside them tenfold (Toby's arrival really pushed Jan on to another level I felt) but as of right now, I don't hold any faith in Dier/ Toby/ Sanchez.