Can't Santon play both sides, so you could buy a left OR right back??
For left-back, I'd strongly suggest Luke Shaw, or you might be able to get fabio from United (Again, can play both sides I think)
He will probably be too exspensive, but one of the best LB's on the game is Alaba.
Can't think of any right-backs you'd be able to get for newcastle, sorry! (Unless you could get the Zenit one! Anyukov or something like that!)
Carl Jenkinson has grown into a 4.5 star potential on my save, and he is keeping Sagna out of my team at the moment too! (Midway through second season)