I believe the Lukaku transfer comes from the need for a Mandzukic-like forward. Mandzukic himself won't cut it anymore, so the management started to look for a player that would be similar to Mario.
Why getting rid of Dybala in the process? Perhaps the problem is Higuain who has almost no offers and keeps refusing Roma. It's just one theory, there could be more but the fact is the management is struggling to get their targets (Icardi, Pogba/Eriksen) and also offload the surplus players.
I'm sure Sarri is not happy with the midfield he has right now so he probably has made a request for an additional CM. Khedira and Matuidi are surley out of his plans, so even in terms of numbers we need one CM more. If so, all the movements would be understandable, you have to somehow finance the buying of new players. The problem is that it's all just guessing at this point, we have no idea what the bigger picture is.