I think that's a bit unfair on Bentaleb and Mason. They were an enormous improvement on the likes of Capoue, Paulinho, Dembele and (sadly) Sandro, who'd been poor in that position for the previous 2 years. I think Bentaleb will develop into an excellent DCM in time (Mason will always be the more attacking of the pivot, seeing as he's a converted #10 anyway).
I wouldn't pin our new solidity just on Dier coming in. I would say:
- Alder over Fazio at CB
- Walker over Dier/Chiriches at RB
- Son over Chadli at LWF (better movement/showing for the ball/creating angles)
have all contributed quite a lot too.
As I've said, for me the solution is to get the Veljkovic situation resolved, and get him in as 4th CB + DM option