The problem isn't restricted to the game time he is (not) getting though, is it? He's a great talent and he possesses bags of skill on the ball, that's a given. And at the age of 22, he needs to develop, both in mentality and attributes, into the player we all hope he will eventually become. But if Mourinho seems more than willing to sign ready-made superstars in order to create a shortcut toward his end-goal (the PL title), what LvG did during his tenure at United (throwing every academy player he could find into the deep end) would not have helped Martial much in the long run either.
It's true that young and talented footballers need more playing minutes. Moreso, they need meaningful game time and not just 10-15 minutes near the end of games that are already done and dusted. But they also need nurturing, guidance and encouragement on the pitch because that's how they'll become more confident in themselves. To achieve this at a club like United is a way more difficult task simply because the standards are always very high. You can't just give them the shirt and tell them to go out and win games, it's not how it works. They must be introduced into a well-balanced side where the more experienced players will provide the platform for the youngsters to work their magic. And as the whole team moves forward, so the young players will grow in skills and confidence and eventually become the leaders themselves when the right time comes.
Take the famous class of '92 for example. G. Neville played at right-back alongside the best keeper in the PL and Irwin-Pallister-Bruce. In the midfield, Scholes/Butt partnered Keane, the natural leader of that United side (alongside Cantona). Beckham was introduced on the right wing while Giggs was already a proven choice on the other side. As for the forward position(s), that's an area on the pitch in which the old gaffer rarely took many chances. That's how you do it. A decade later, when the young and still unripe Ronaldo came to United, we didn't rely on him to carry all the weight upon his shoulders straight away. When his time finally came, it was clear for all to see and not a matter of debate on a football forum.
Our main problem and Martial's main problem too is that we're not a good football side. And it's not solely a Mourinho vs youth issue, we're not a good football side for quite some time. You may cry all you want about how Martial, Rashford and <insert young player's name> should always start and be trusted but i'm afraid it won't work out well. Not for us and not for them. It didn't work for Moyes when he was desperately waiting for Januzaj to become the answer to all his woes and it didn't work for LvG when his philosophy crashed and burned and he was searching for a lifeline in the academy.
The club must help Martial in order for Martial to fulfil his potential and help the club in return. But this can not happen in an environment where Martial and all the other youngsters will always start and it'll be ok for the club to fall in midtable mediocrity because of the hope that in the future these players will deliver the goods. That's a first class ticket toward sheer mediocrity and the total Arsenalization of the club. And trust me, when that happens Martial's departure will be far more certain than it seems nowadays.
SAF was the master of rotation but he's not here anymore and i understand the question marks regarding Mourinho's ability to rotate and therefore give an active role to the youngsters. But i think his initial idea to rotate Martial/Rashford at the start of the season was correct and bore some fruit. After all, Martial gained confidence through having a huge impact as a sub that could change the course of our games. He may get there eventually and perhaps not under Mourinho but he'll benefit more from being in the rotation for a good side than simply starting every game for us now. Unless, of course, he finds guaranteed game time in an already readymade side like Real Madrid, Barcelona or Juventus. But i'm not sure this will happen, at least not this summer.