I don't think they'll throw money at anything, especially if O'Neil is still here. They'll buy a CB but only if Sá goes for a decent amount and they could easily sell someone like Cunha or João Gomes as well. I'd also expect them to offload Sarabia and/or Hwang if O'Neil continues not to use them.
They will know that relegation is a possibility (even an inevitability) with the way they've been operating and they won't be as afraid of it as they were a few years ago after seeing some of the fees teams like Burnley, Leicester and Leeds have brought in for some of their players.