I think our decline was progressive and the system we played went to pot after Gueye left. Obviously I am talking with a great deal of hindsight - I actually thought Carlo had lost the plot for the second half of the season, but in reality I think he just had us playing out of our skin for the first.
Ultimately, our squad has been hideously balanced for some time. Midfielders haven't been collectively good enough for any system you could conceive of, we haven't had the pace up front to justify playing deep enough to accommodate our slugs at the back and when Godfrey looked like he might help for a while under Carlo, he has regressed as well. We've been a perfect storm of complete mismanagement and while I'm not naive enough to think this window is going to be the one that rockets us up to mid-table, we at least look like we've identified a way of playing (however unpalatable) that we can accomplish with the squad we have plus some key reinforcements.
If Onana's cameo is anything to go by then we're already a good way towards solving the problem of getting the ball up the pitch. We've got a couple of meat and potato centre halves who'll get us back to doing the basics well and HOPEFULLY we'll get some pace and guile up front.
Coady and Tarkowski are not long-term solutions so it may be that 5 at the back is a stopgap en route to a bigger transformation.