He scored 23 goals last season without being the penalty taker, and he missed 13 games through injury. He could have potentially been looking at 30+ goals last season without injuries or if he took pens, and that's not in a great team either. Thinking about it like that I think we're potentially being a little harsh due to his bad start to the season.
What does disappoint me though is he doesn't seem to have developed the poachers instinct you want from your number 9, too many games where it doesn't really look like he'll get on the end of any chances or really look like scoring. His finishing has generally been good since we signed him which is how he scored a lot last season despite not really looking like a consistent goal threat. However the rare occassions where he has looked like a goal threat this season his finishing has also been woeful.
I'd drop him for Cavani for a few games or potentially drop Rashford and move Martial out wide.