I'd sell Debuchy on the hope that Carr could find someone better for peanuts. Think Mathieu is alright overall, good movement going forward off the ball and has a very good recovery tackle in him, but he also loses his man alot (hence why he's frequently playing catch up) and usually does nothing noteworthy on the ball.
As Ronaldo has posted earlier, think we'd be better off with someone solid at the back first and foremost. Beye at his best was miles better than Debuchy imo, he'd lock his flank down defensively and pretty much shut out anyone on his flank, and wasn't too shabby going forward either.