If it's a fail, fail and more fail, yes, sell is the better. If you have a player programmed to do well asistances, or score, he will do this action always, if he fail, one time and another, that's the way he will bring. If you want you can try to put a picture of this MC that is failing, and too about the formation, but you know my viewpoint about all this.
Of course is interesting have the your 8% possesion + the 5% in your stadium.
My opinion at now is that you have to sell your worst MC, and try better luck.