Imho the drop from Calabria to Conti is the second biggest problem after the lack of a quality RW.
Tonali and Pobega at least gives me some hope that they will improve tactically in the DM position and have some kind of sudden leap in quality we already seen in other young players, like Calabria.
Calabria's absence was clearly felt every time he was out, but I think it was mostly for tactical and cohesional reasons.
The RB is a crucial "balance player" in our system, and it is currently molded for and by Calabria, leaving his substitutes(Dalot, Kalulu) with a very difficult task whenever they're asked to fill in for him. However Calabria himself used to be on the other side of this issue. Conti was the starting RB and "balance player" until he got injured in July last year, and we used to look unbalanced and disjointed whenever Calabria substituted for him. It took a while for him to adapt to the team, and for the team to adapt to him.
Don't get me wrong, I think Calabria has proven to be a more dependable "balance player" than Conti at any point, but I don't think the issue has as much to do with quality as it's made out to be.
The performance drop from Kessie & Bennacer( and heck even Meite) to Tonali on the other hand...
I think that one actually has a lot to do with quality and characteristics. Tonali is simply not ready for a pivotal role. He has talent and versatility, but I think he's more likely to excel at the Krunic tasks, as "half-winger", box-to-box runner, auxiliary playmaker and auxiliary DM.