A single pivot like this can work, as long as all (or at least a significant majority) of the other surrounding players step up and contribute to helping the midfield. If we ever get Shaw, Martinez and Dalot all on the field at once then that's most of the backline who are capable of playing their part. We'd still need a right centreback who can have more of an impact on the ball than what we have, but the backline as a whole isn't too far off theoretically working the way it should.
The big issue is the players we have ahead of the pivot. Bruno gets involved quite a lot, but he's obviously not the safest on the ball and his pressing is chaotic (which sometimes works in our favour and sometimes works against us). The player in the second #10 / attacking #8 (whichever you want to call it) is a much bigger problem though. Scott's obviously the one who's played it most and he abandons the midfield far more than what someone in that position should be doing (while also regularly being lazy at getting back defensively), while Mount wasn't any better in the few games he was fit for. Combine that with our wingers also not helping out (Antony holds onto the ball more but doesn't do much with it, while Rashford and Garnacho basically just attack the fullback every time) and we just end up with a very bad shape.