I'd say Davies is also unsellable for him - personally, I suspect we'll be seeing Davies in that left-back slot as the first-choice for the entirety of Mourinho's tenure here.
He's the steady-Eddie type that Mourinho absolutely adores in one of his full-backs. Paulo Ferreira at Porto, Ferreira/Gallas at Chelsea, Javier Zanetti at Inter, Alvaro Arbeloa at Madrid, Ivanovic at Chelsea...there's historically been a role for a defensively-minded auxiliary full-back in any Mourinho side, to counter-balance the marauding other full-back in Cole, Maicon, Marcelo, etc.
Davies fulfills that role to perfection, with the added advantage of being a half-decent wing-back if the situation calls for it - if we're chasing a deficit, he can be sent up to act as a wing-back, and will likely do better at it than a centre-back playing out wide, without needing a substitution to enact that tactical change.
The way I see it, the elements of a Mourinho team that we'll need are the below, in a rough 4-2-3-1 -
Attacking RB-------Limited, Pacy CB-------Ball-Playing,'Leader' CB-------Defensive LB
----(?)-----------------(Sanchez)-----------------(Toby/Dier/TBD)---------(Davies)
-------------------Defensive Distributor-----Ball-Winning Midfielder----------------------
------------------------(Lo Celso)-----------------(Hojbjerg?)-----------------------------
Pacy Inside Forward---------------Creative AM-------------------------Pacy Left-Winger
--(Lucas/Bergwijn)----------------(Alli, possibly Tubby Ndombele)----------(Son)-------
----------------------------------------Kane-------------------------------------------------
---------------------------------------(Kane)------------------------------------------------
The bolded bits are the big question marks atm, imo.