I don't completely agree with your opinion, if you say Juventus' defense needs to be improved then that is the right thing. however, Juventus currently still has a very good defensive quality but the coach has not yet found a suitable formation.
It depends on how you measure it. If you look at the Serie A table, Juve has 5 goals against, not the best defense in the Serie A as Inter only has 2 goals against. But of course, it's good but not really the best in the league.
Sarri needs a good DMF so they could decrease the goals against for each match.
yes, but at least it's still not too bad. I agree Sarri needs a quality DMF player like Arturo Vidal, that player already belongs to Barcelona. it would be great if Sarri would later try to bring Kante from Chelsea.