I feel pretty comfortable writing off Campbell too, and it's not based on whatever small amount he will or won't contribute with this season. It's more the fact that he's not worth persisting with. Welbeck is the same. Neither of them are good enough. Take whatever you can get when they absolutely have to play and forget the rest. Their presence is a nuisance.
Conversely, injuries might be dangerously close to finish Chamberlain at top level right now, but he has more talent in a fractured toe than those two do in a pair of healthy feet. Him I'd persist with for a while longer.