It's easy to see that he is simply not suited to the PL pace at highest level. Player that's good on the eye but has nowhere near enough other attributes to play at high level for United, I am not sure he is even good to come as a sub these days. There are lot of players in the world that have good technique but can't play in PL.
Even Berbatov was always bad as a sub, but Berba had lot of other abilities such as passing, good shot, finishing, football brain, etc. and I don't think Zirkzee will ever be on that level.
We should clearly move on and replace him with someone who can bring more intensity in second half.