Yes, that's it more than likely. He looks like a winger doing a job rather than a natural defender. While other defenders are refining their art at 24, Danny is still learning it IMO. I think it may be too late for him to re-task himself as a defender though maybe I am being a little hasty. I hope so.
Good coaching will obviously help but I just don't see any defensive traits there. No raw material so to speak. As a defender you need to want to stop goals against first and foremost. Bust a gut to get back in position like Walker, or throw yourself in front a pile driver like Dawson, or read the danger and snuff it out like say Vertoghen (used to). I don't see any of those qualities in Danny and it might be too late for him to learn them now, or at least develop them to a sufficient standard for a CL chasing team. Anything a FB contributes from an attacking sense is a bonus really, though fast becoming a requirement I suppose, but if you don't have the basics of defending right then those attacking traits are the icing without the cake.