Frattesi is a creative midfielder. Well maybe not creative, but definitely an attacking midfielder. I have been watching Sassuolo recently and will pay more attention over the next few months. Frattesi is in a similar mold as Barella, though better with his shooting, and worse in defense or covering for others. The latter is the tactical development he needs, and it'll be interesting how he shapes up for that.
Having 2 of Barella, Calhanoglu, Frattesi always starting is enough attacking potential in the midfield. The rest has to come from wingbacks or forwards, which is where Dybala would be perfect for us.
Back to midfield, what we would need is more solidity and an able backup. Nandez could come in there. With the wages that Brozovic, Barella, Calhanoglu will be on, surely the next midfielder we sign would be for depth and more cover. Nandez fits the bill perfectly, after Frattesi who will be the first backup.