I see Bentancur as that kind of player, but he needs someone to guide him and help him achieve that potential.
I think we'll play 4231 with Costa, Ronaldo, Dybala and Berna.
I see Bentancur as a young Khedira. Does the little things well, great poise, highly intelligent, plays simple, can go forward to sneak some goals (though he hasn't shown much willingness to do so). He isn't going to break a game open by making opportunities or scoring goals out of nothing. When Khedira was playing his role to perfection, he was an integral part of the midfield. So in my eyes Bentancur as his successor is indispensable (plus he has a lot more potential to fulfill given his age).
Matuidi seems like the obvious omission but he brings something unique of his own - that huge amount of energy and relentless pressure which (as I keep alluding to) is what we were fortunate to have with Vidal and desperately need. We need to replace him with Ndombele who brings that same kind of dynamism but with amazing skill to boot. SMS, potential world class though he is, won't bring that energy and athleticism which is sorely needed. Rabiot is more in the SMS caste minus the eye for goal.
Can is our poor mans Mascherano and I would like to see if Sarri could manage to play him in Pjanic's role but it's doubtful considering his penchant for Jorginho in that position for whom Pjanic is a like for like.
Matuidi for Ndombele would best round out our midfield. Fat chance though from what I'm reading.