Arsene very recently came out and said that he had total Control of his technical staff so I assume that means playing staff too.
The board would never sell a player without Arsene's consent, I'm pretty sure of that.
MO & ItsMe have pretty much covered my main concern if Nasri goes, Arsene will try & replace him on the cheap or probably not replace him at all and defiantly state that Vela or some other project needs to develop.