You are making a great point but hear me out.
point a - First of all, our most natural FBs are attack minded (Shaw and Valencia). Unless, José doesnt fancy Shaw, or doesn't want him to play his natural way (imo), should he not support to allow the player play to his optimum capacity? Playing Perisic could mean the end of Shaw as a more attack minded FB.
point b- Secondly, unless José is going to heavily utilise a #10 that has a eye for driving in and getting goals ala Micky (or and James), I feel like the goal scoring burden on Lukaku will be too much. First of all, we have close to zero options on the RW, where I feel we need reinforcement but since Valencia will bomb forward, it makes sense to have an IF to join in the box to augment Lukaku's scoring. I hope we get James but I feel like a DM will be a priority too given that we have none. This is how I imagine it, a 433 with one DM, say Fabinho that acts like a false CB falling into psuedo back 3, 2 FBs bombing forward, 2 wingers cutting to flood the box with Lukaku. I say this because we made chances but had no bodies in the box.
point c- This way, we make the best use of every player at our disposal, something Chelsea did well last season. Perisic will effectively end Shaw and Martial as we know them. It isn't really worth it imo. Martial has a thirst to be in the box and an eye for goal more than being a supplier, let him flourish and be the player we hope, Shaw wants to bomb forward and while I know he leaves spaces, hopefully our CBs are better plus the addition of a DM. Lukaku will most likely be used as a target man, let him be the guy to lay it off for runners into the box, which we lacked last season.