Got to be Nacho. Nasri might not be back in time and Nacho will be an excellent threat off the bench, as he's already shown.
This season we've seen the likes of Demichelis, Kolarov, Sagna, Mangala and Navas used quite regularly with all the injuries we've had. De Bruyne, Sterling, Bony, Delph, Otamendi and Nacho have also had to be used despite needing to settle in the team. Added to this is the inconsistency of Yaya and we're not showing title winning form.
What's brilliant is despite our best 11 involving the likes of Clichy, Zabaleta, Kompany, Fernandinho, Silva and Aguero (6 players in addition to those above) we're in 4 competitions.
We've now got to balance things as best as we can.
Other threads have talked about players warranting a place in the starting 11. For me, away from home we play with Sterling and Navas out wide and potentially Delph and Fernandinho for energy in midfield with Silva behind Aguero. Bring Yaya on for composure/quality later on (De Bruyne is a different player away from home). At home is where De Bruyne and Yaya can come in, where the opposition may sit back more and their quality will see us through.
Anyway I've gone off topic. Get Nacho into CL and let Nasri come back and see us through to add the league and FA Cup as well to the League Cup and CL that the rest of the squad win.