Calling him up early last year would have been being rushed. That might be ok for some players, but it might destroy some other players. Some guys struggle for the first time, develop some bad habits, and that's their career. Some guys might thrive with that challenge.
Calling up Montgomery now would not necessarily be "Rushing him" in terms of a schedule you might have drawn up when he was drafted...but my word is this a player who did not at all look ready for the big leagues for an entire season at AAA. What would calling him up last year have done to him other than embarrass him?
I don't care about the schedule when he was drafted. I do care about the fact that he couldn't handle pitching at AAA and major league pitchers blow AAA pitching out of the water.
Rick Hahn's thing, one of his nonstop things, was calling guys up not because they deserved it based on their performance but because of external factors. Moncada, Giolito, Lopez, were called up because trades created vacancies on the big league roster. Madrigal, Vaughn, Rodon, Anderson, Fulmer were called up because we desperately needed help filling holes on our so-called competitive teams. Robert, Jimenez were called up because they signed their contract extensions. How much did all this backfire?
Treat guys consistently. If they do their job and perform at a minor league level, call them up when the opportunity presents itself. If guys struggle, they have to overcome their struggles before they move to the next level. If you don't want to do that, then just bring back Rick Hahn, he was masterful in making moves for reasons other than baseball performance.