If he choses Sanchez and Dier together, and we don't win, then he only has himself to blame. He can't throw any of them under the bus. We have all seen the goals we've conceded due to the both of them.
There's no reason to be playing Dier and Sanchez together, ever. We know it does not work so it's time to give other combinations a go.
I think Toby is still our best CB, but unfortunately time is against him now. I like the look of Rodon, but he needs minutes in the league to see if he's got real potential. I'd also be happy with Tanganga playing CB as I think he's better there than at RB.
Basically we have options so like you say, if José doesn't use them and ends up out of a job it's his own fault.