Problem is there is talk we are wanting to shift out the likes of Willock and Nelson - if so then they are not an option.
If we fail to sign anyone creative or progressive for either CM or AM then that leaves us with the likes of Martinelli, Nketiah, Willian, Pepe, Xhaka, Luiz, AMN, Cedric, Mari & Runarsson our bench/rotation core.
As such I don't think the 4231 is sustainable at an effective capability without the right signing/s.
Accordingly I'd look to change formation to something we do have the capability to sustain = 442.
Bottom line I can see the priority to shift players out coming back to bite us if we do purge but then fail to add and/or then compromise by a panic buy/loan much the same as seen with Cedric, Mari, Willian, Luiz, Sokratis, Lichtsteiner, Runarsson, Xhaka, Mustafi, Kolasinac.