I agree with the posters who say he's being ignored a lot. I feel like the only way a Dutch player can enter our dressing room with some status is if he shines at an international tournament. Goes to show how big the gap is between the Eredevisie and the Prem. Pröpper was a goal scoring gem in the Eredivisie and he's on the bench.at Brighton with only two goals in 100+ games. Depay would probably have been a solid player for us now, but he needed too much time which he (rightfully) did not get. Those were easily the top players in the league before their move to the EPL.
Donny is unlucky the Euros got moved. He would likely have started as the 10 with Wijnaldum and Frenkie de Jong deeper in midfield. De Roon has looked very poor on the ball in his appearances and would likely not have played the easier games in groups where the Dutch are likely to dominate possession. Now that VDB is on the bench it's likely he won't get a call up. Leaving benched players at home is a much more common practice for the Dutch NT than for others. De Boer did the same with Strootman in the last international break.
Reminds me of Van Persie's situation at Arsenal. When he came in, his deal with Wenger was that he would get cameos here and there for the first two years. According to Robin, it was only after his return from the 2006 world cup that the dressing room started taking him seriously.
As some posters have pointed out, he wasn't Ajax's best player. Those were De Jong, De Ligt and Ziyech. VDB was still the best of the rest. Now it could very well be that's not good enough for United, but people saying that he was average or that he looked like a 'nothing player' for them are just rewriting history. He scored very important goals for them, had the best link up play from anyone on their team and rarely lost the ball (which is a more highly regarded skill over there, especially in the centre of the pitch)
I don't believe there's anything Donny can do in training to change his current predicament. He'll need an extended runs of games to prove he can come good and Ole has made it abundantly clear through his actions that the only way he'll "risk" playing Donny is if injuries hit.