Yeah I agree with that, their whole system is high risk because one wrong decision can let someone through on goal. It can be devastatingly good when all the parts click but it only takes one mistake and you've conceded a goal. Signing a quality CB will definitely help but really they need all of their defenders, all of their midfielders to be at that level defensively if they want their defensive record to improve.
They will be a force if they sign a decent keeper.