I watch a fair few PL games and has probably watched Newcastle 6 or 7 games last season. Personally, I prefer FBs to first and foremost be good defenders, so naturally I rate Sagna as a clear level above Debuchy. Having said that, Debuchy is far from poor when defending! He is quite a bit more inventive in attack than Sagna; I'd say that Sagna is only better than Debuchy at crossing when it comes to attacking. However, I like that Debuchy often makes runs in behind the opposition back 4 and crosses from deep while on the move. It generally causes more trouble when defending against IMO. Sagna generally crosses from a standing position and in front of the back 4.
All in all, I think Debuchy is a solid signing. We will lose a bit defensively, but we will probably gain a bit offensively. Hopefully that sort of cancels things out.
