There are serious doubts about Mount the CM for me, but what isn't in doubt for a single second is his speed of thought, tactical understanding and ability to exploit situations. In fact, before any physical attributes, this is the primary for Mount - there are few who see and react to the happenings on a pitch faster than him in the entire league, let alone in our team.
He's a player managers will always find a use for because he'll find a way to adapt whilst still offering attributes everyone wants in a team, namely dogged determination, adroitness and the discipline to go again and again with full concentration and no lapses.
Before we factor in his physical attributes, we should look at his guile and focus - he remains unperturbed even when having a torrid game; he doesn't flap, wilt or fade; he's a guy you can bet on to to be in the running to take on a chance in the 95th minute where others are already switched off and/or exhausted; he's the guy still running when everyone else has stopped. His physical qualities aid this, but primarily it is intelligence and mental fortitude he has in bucketloads and what affords him the potential to be a resounding success here in terms of effectiveness and bang-for-buck.
He is the antithesis of a player like Van de Beek - by their very natures VdB is not equipped to do what Mount does - Donny wilts in pressurised environments, Mount thrives in them. Van de Beek might even be tidier, more technically able player in terms of passing and connecting play, but he has no nerves at this level of play and I'm not sure his mind can keep up with the pace things need to be done at for him to be effective.
On top of that is the character to fail or make mistakes and not let them affect your game. Mount's head won't drop and he'll be just as determined from one play to the next; we have very few players with that kind of constitution and I would expect Mount to make a name for himself popping up with decisive actions late in games where others are giving up. I'll never expect the same of VdB.
Mount is a perfunctory player; no thrills, very little flash, but extremely effective. In fact there's an irony to his understanding of play being fans will chide his finishing yet fail to recognise he read an entire play and worked himself into the position for the take on, which he is then being criticised for. He has a lot of subtlety interlaced with the grunt work, and if we can get it to click, there are very few in the league who can replicate his combination of physical and mental attributes.
I am not a particular fan of the player nor do I think he's the best use of funds, but there's no question at all that he can make a name for himself here if correctly utilised and become an essential cog in the machine to the point we have nobody else who can fill his shoes when he's absent. The question is whether we'll draw that out with the proposed system and positioning or whether it'll collapse and need amending. I have serious doubts about his raw quality as an #8, but I do think we and he will find a way to adapt and optimise if things aren't working initially and there aren't many players with the capacity to do that.