See I agree you can't make a total judgement on a player with these brief cameos, but they usually show you a glimpse of something. Whether that's their hold up play, pace, strength, positioning, passing ability, work rate etc.
This lad has shown none of that and it's quite worrying. I'm also quite concerned after nearly 9 weeks of being here he's still not fit, with Martinez trying to clearly protect him in the media with this wrist injury. Journalists in Russia and the man himself cited how hard he works as one of his endearing qualities yet his substitute appearance at West Ham game he trundled around like he was playing with his kids in the back garden? At least yesterday he ran around, albeit giving it away and falling over at every opportunity.
I don't think anyone is writing him off at all, but the fact is he should be nowhere near the squad never mind the pitch at the moment.