I wonder if the plan is to go a back three. At the last game of the season, Hughes seemed to take a leaf out of the Guardiola/Bielsa book and put Draper at the back as a kind of ball playing centre-back. I'm can't remember where I read it, but I'm sure I read somewhere once that it's easier to retain possession with three at the back than with four. As we've seen at the world cup, a back three or a back five with wing-backs seems to be coming back into style (Chile, Argentina, Netherlands, Costa Rica, Mexico have all done it at some point as well as probably others).
As others have said though, we don't need another centre-back. Warren, Meekings, Devine are all strong capable centre-backs and Meekings is a good distributor in my opinion. Tremarco also played well there in one game and Raven, Draper and even Foran could all fill in there if needed, as could Jason Brown. Shinnie could probably do a job in there as well. A new right back would be nice, but by no means essential. Raven is obviously the first choice and Meekings and Shinnie are capable in there as well. Warren can also play there and Devine could get away with it if we really needed to (he did have a shocker there against St Johnstone in the 4-0 game though). Seemingly, James Vincent can also fill in there in emergencies and Tremarco could probably get away with it as well. If we can, we should use whatever funds we have to bring in another striker - I think everyone's in agreement on that and after that, using it to retain members of the current squad. A right back would be nice, but not essential and a new centre-back is not needed at all, though if Hughes can get one for little money on loan, then that's not so bad.