I think Sarri is looking at Bentancur's role in the wrong way. He can sub in for Pjanic but he is in no way a like for like and instead a DM in the mould of e.g. Busquets who is pretty good at intercepting and recovering the ball.
If we play him instead of Pjanic, we need more creative players as the mezzala's. We can't just put him in, play the same way and expect the same results.
Bentacur has some problems with tackles or controlled aggression. Both are vital in any DM. He needs to be calmed down, but I fear his lack of playing time is whats the real issue here. His career might go from top midfielder to a Juve reject in a few games and he is, I suppose, extremely tense as a result.
Very difficult situation and our recent transfer activities doesn't help either. But if we want to be a top team we have to field those who are playing better and have a deep bench as well.
I think Bentancur would be a good water carrier type player. Maybe he doesn't really excel at anything in particular, but he does multiple things well. Keeping possession, being tidy, reading the game, cutting passing lanes. He seems to read the game well, too. Not going be some superstar playmaker or b2b, but if you stick him next to a creative player and a good b2b or destroyer, would definitely be useful. He's still pretty young so no reason to sell him.