I haven't seen many United games tbh, but from what I've seen it looks like you lack attacking threat from wide positions. You have good players in those positions, but they lack that ability to beat their man in 1 vs 1 and curl the ball in the far corner, like a top class winger does. And I'm talking about doing that when the other team is set firmly in defensive positions, not on the counter, where any good player with pace can dribble and score a nice goal. But in positional, you need more players like Neymar for example, or even Alexis Sanchez. Who would receive the ball in a neutral position and individually destabilize opponents. Right now, almost everytime one of your players is in the wings, the main course of action seems to be to put in a cross for Ibrahimovic to use. That is not a very durable tactic, nor a very modern approach. You basically play the way most teams in England know how to defend.
Martial, Mikitharyan, should be players who score 15+ goals in a Premier League season. How many did they actually score? And why don't they score enough? Imo, this is down to two, or even 3 factors. 1) Their inability (atm) to consistenty turn neutral situations into goal scoring chances (either for themselves, or for their teammates, by way of assists or pre-asssists), 2) Because you do not have a first class playmaker, who could actually find clever passes under conditions of limited space, and 3) Because from what I've seen (again, I might be wrong), Mourinho tactically limits the players' movement to a pretty confined space in positional play. And when you, as a wide player, are instructed to stick to your designated area, the only way to score, or provide chances to your teammates is to possess the abilities and qualities from no1. Basically, you're playing Martial for example, the same way Luis Enrique is playing Neymar in positional play, but the difference is Neymar receives the ball and most of the time he would create havoc in his side, thus creating a 'hot-zone' for the opponent, forcing him to make a move to close that danger, which of course, leaves spaces other players can use. This is one of the classic ways to create superiority, you put your best players on the ball in positions where losing the ball is acceptable (which is the last third of the pitch, in wider positions), and you ask them to attack their opponent. And this situation usually draws some sort of reaction from the defending team, which you can, in turn, speculate. Either that defender is left alone to defend your attakcing player (which ends up in 1 vs 1 that your player can win), or the defending team brings more players around your dangerous winer, meaning more space is automatically available in other zones, and is up to the attacking team to find that space.
Basically, when you choose to play a positional game which doesn't let wide players move around alot, you force them to individually solve duels. But if they aren't great and consistent at that, you end up with less danger coming from those wide positions. Out of curiosity, how many games have you won in which Ibrahimovic did not have an important role? If the number is low (I suspect it is), imo it's because at least 1 of those 3 reasons I mentioned. If you play a more static positional game, you have to have players who are able to consistently beat their markers. You don't imo. And this is why you are basing a lot of your attacking plans on Ibrahimovic. When he has an off game (remember that he is 35), there are problems for you.
For me, again, from what I've seen, it looks like Martial is not better than he was when he was playing for Lyon. He showed flashes of brilliance in France, like he does at United as well, so that is something that was there in the first place. He's still very young, so he can become world class for sure in the next 2-3 seasons. Problem, for a team like United, is if you start using a position to raise the level of a player so that in 2-3 years you have a world class option there, you better make sure the other positions are filled with top players. But they aren't. You are even demanding more from Martial, because even if he is not nearly the finished product yet, he is still one of your better and more talented players. And that is what needs to be changed. Promising players don't have to consistently play under pressure. The pressure is supposed to be on those established world class attacking players. In this way, your 2-3 talents can really grow.