Allan is the Vidal type midfielder that we have been sorely lacking. High energy defensive midfielder with technique and passing. Exactly the kind of player you'd want to support Pjanić.
If we get Allan, regardless of the formation, you can bet Sarri is going to start him from the get go.
I would also add that our chances of winning CL increase with a signing such as this but I highly doubt this is going to materialize anyway.
Allan is very far from Vidal, he has tenacity and can actually pass a little, but Vidal was complete package. Allan's offensive contribution is very small, he rarely if ever scores, I don't think he makes more then 3-4 assists in a season. Vidal was an offensive machine, that what separated him.
In 4312 we need our LCM and RCM to be able to drag the ball, dribble, move a lot. I don't see Allan doing it.
In 433 he might start alongside Pjanic and possibly Benta.
tl:dr I don't think he is a deal-breaker for us, but he will be a welcomed addition