Therein lies the problem. The common denominator is that every defender we buy ends up looking shite. Yet, some move on and look half decent (see Savic). If the system, and other defensive personnel, aren't right - we're already fighting a losing battle. The shortsightedness is incredible, it really is, for an establishment that is supposedly high up in the world echelons of professional sport.
I don't really see a pattern in our signings - it seems like we just buy the latest shiny product. We buy Sterling, superb in a counter attacking Liverpool system, and then proceed to play keep ball - the same with Sané often (although he stills looks to beat a man.) Mangala, a dominant physical centre half, and ask him to play keep ball. Either sign players that suit how we want to play, or change the system to suit the players. Granted, these players especially Mangala probably were;t good enough anyway - but asking them to play alien systems to what they are good at, is either blind arrogance or ineptness.