Sagna will probably go to PSG if he leaves
As far as replacements go:-
Expensive ones: Coleman, Piszczek
Young but very talented: Chambers, Clyne
Been around but could do a job: Richards, Juanfran
Internal solutions Bellerin
I'd try for Coleman failing that I'd get Calum Chambers - he looks a serious player. Athletic strong and good on the ball having switch from midfield