Are those of you who say that he should suck it up/get over it, "it's just a number", intentionally missing the point?
Come on now, you realize that it's not about that at all, right? Martial could have any number and it would be fine, it is not about the number itself. It is about what the number change implies. It is just a show of utter disrespect. "Zlatan came here, he is entitled to pick any number he wants, even if it is yours, because you are seconary to him. Frankly, we just do not care about you as much as we care about Zlatan, so we will give him the number he wants, even if you want it too. Your wishes are not treated like Zlatan's wishes, he has priority over you". This is LITERALLY what this change signifies.
It is not about who is more skilled. I am pretty sure that Martial himself knows that Ibrahimovic is much more experienced and probably a more well-rounded player, since Martial is still very young. This is about feeling valued and respected. Up until now, the 9 was Martial's number - it is his, why would you take it away from him? It is unimportant whether he not he ACTUALLY cares about the number, the fact is that he MIGHT POTENTIALLY have a special bond to the number that he picked for himself when he came. Therefore, taking the number away from him could hurt him emotionally, even if it is stupid - and the clubs knows it. The fact that they still do it without asking him first shows that they frankly did not care enough about Martial's feelings, they'd rather placate Ibrahimovic.
All in all: The number was Martial's. We don't know if Ibrahimovic insisted on getting the #9 or if United wanted him to get the #9 for marketing reasons, but the fact of the matter is - taking a number away from a player might hurt/annoy him. Not asking him whether that is the case shows disrespect, and that that the decision makers clearly care much, much less about Martial's opinions than Zlatan's.
This is crazy. I did not think Utd would do this; at my club, we don't even take away numbers from fringe players - Hofmann had the #7 for two years and we did not ask him to take a different number. It was about showing to him: You might not be a first 11 player right now, but we value and respect you just as much as anybody, you get your fair chance, and if you play better than the others, then you'll get selected.
If I were Martial, I'd read this move as: "We settled for Ibrahimovic as our focal point of attack, we'll build around him and market him as our man. Even if you outperform him, we just care more for Ibrahimovic, so - sorry, but you cannot be our star/best player/first choice striker.", because it means that undoubtably, Ibrahimovic's needs and requests come first, and Martial gets what is left.