I think the right comparison is with Berbatov who many on here claimed was "too slow" and "lazy" back in the day.
However, the challenge with that is Berbatov came to the EPL at 25 (a massive difference). Then he had 2-3 years at Tottenham to get used to the EPL before coming here. Back then, Spurs was in a place where -outside of wishful fans- he had time to adapt, the pressures weren't there like they are today at United where fans, media, owners, coaches, etc are all hoping to show everyone...hope.
Frankly, if we believe he's the next Berbatov, the only way he becomes that at United is if we loan him to a smaller EPL team where he's guaranteed time with lower pressure. Otherwise, at this rate, we'll break his mentality so much he'll end up being transferred to the Greek league. Sadly, we're not in a position where we can spend so much to just loan out a player-as-a-project.
Garnacho is someone who is still a project but who's contributing as he's developing. I don't see that from Zirkzee at this point, in our weakest position.
For that reason, and even understanding he could become a top tier player, we have to sell him if we think he needs up to two seasons to catch up to the speed of the game. Now is more important than "then" for a player who can't contribute today.