We started Pogba in the first 3 games, so your insinuation is that VDB will also be a defensive liability like Pogba is. Whilst VDB has most experience in a 10, he's actually very capable of a box to box role - and his pressing is actually one of his best attributes. I genuinely think he has all the qualities to play in a box to box role but we haven't yet seen him as such.
I don't know what impact Donny will have on this team, I actually cannot tell you that. I have not seen him play enough to give my judgement.
However; I have watched enough of United to tell you this:
Because of how we started the league, yes Ole has to take the blame for playing Pogba instead of Donny at the start because Pogba was not fit. That is the past and it happened.
Ole cannot take another risk playing Donny in that position in this team with the fixtures we currently have, its too big of a risk. I am quite certain that he will experiment once we are past these fixtures.
You cannot blame Ole for not picking him when we have conceded 9 home goals and he chooses to go slightly defensive the next game after being battered 6-1.
He may well have the ability to play that position, he has come through the Ajax system so is adaptable but I am not blaming Ole for not taking the risk at the moment. Once the defence has some more confidence, he will come in.