Can't Pieters play in other defensive positions? If we can't have two players for each defensive position we could do with someone like that.
I've also theorized that Pieters might possibly be a "Perch replacement" (a versatile defender) and we might be after a second left back, but given past history, it does seem unlikely that we'd shell out 6m+ on two defenders with one of them not even meant to be first choice.