Most players need time to adapt but you can usually see within a few months who is not going to make it (which is why I am worried about Pogba, doesn't look much better than the kid we let go), de Gea always looked as though he could adapt.
I never dismiss players without reason, I told you all Can was not up to it and now it seems some agree (though I was a wum at the time), I said Coutinho was a 1 in 5 player again some now seem to agree, I said Firmino was a 1 in 8 player (not his fault, he is a great second striker used wrong) and some would now agree. I have never dismissed Matip (even though he has had the odd dodgy game) because I can see he will make it at the top level, in fact I have never called most of your players (certainly not as many as you) because I rate most of them quite highly and just think many are doing a job out of position.
I get a lot of stick as a wum on here for telling the truth just because I am a red manc, some know when I am serious and agree or discuss with me, others just stay in the safety zone by calling wum.