The first goal is the most important. Mourinho knows if we can get the first goal we're probably going to win the game. If that's by defending the 1-0 (Southampton) or blitzing teams in the last 10 minutes (Everton, Swansea, West Ham etc). The first eleven and the team that starts the game is therefore tasked with the most important and difficult part of the game. We COULDN'T break teams down last year.
After we take the lead and our opponents change shape trying to chase the game, we react by either getting Martial on to exploit the space, or bringing on Herrera/Lingard to close the games. I don't get how people are coming up with the notion that Martial is better against packed defences. Cutting inside and doing the, at this point, almost trademark dribble he does is easy to defend when you have 8 defenders. I keep talking about this context but people bang on about him coming off the bench and scoring as if the game hasn't been changed by the first eleven (and Rashford) by then.
If you want to forget that Martial failed with this type of movement time and time again last year then that's on you. Mourinho clearly hasn't. Rashford starts and we're getting the first goal and winning games.
This is how I see it as well. Martials game is too one dimensional to work against a parked bus. You beat that kind of defense either by being clever and creative with your passing and movement around their wall of players(this is where Mata and Mikhi are useful), or just pass it out wide and keep banging in crosses. You also need players who can shoot from distance to force some of their players to come out. Keeping the width is very important as well in order to stretch them and keep them from congesting the middle.
Some will argue that a dribbler like Martial can be useful in these situations if nothing else is working, to take on players and create confusion among the opposition. Which is true but it's generally very difficult to get through a parked bus this way. Especially when you combine a player like Martial who doesn't stay wide with a FB who isn't up there to keep the width.
On the other hand, his strengths make him ideal for counterattacking. Which means he should be suited to start against the big teams when we don't expect to have much possesion. But the problem with this is, that he would be tasked with helping to defend on the wings, and this means he would be spending most of the game defending, which isn't one of his strengths.
I think the main problem for Mourinho here when it comes to Martial is because of tactical reasons. He probably feels that he has to make to many adjustments to accommodate Martial, which he is obviously reluctant to do. I think the reason Martial is only competing with Rashford for that LW spot, is because Mata and Mikhi are different kind of players. They bring something to the game which Martial doesn't. Rashford and Martial are similar type of players, so naturally, Mourinho doesn't play them together.
I can only see them starting together in matches where we're playing on the counter against teams that offer more threat, as I don't see the need for Mata when we're expecting them to have most of the possession. We haven't played any top team yet so I guess we'll see if that happens against Liverpool. I wouldn't be surprised though if he went with Lingard instead of Martial, or maybe loose Mata for Lingard and play Martial-Lingard-Rashford. This way Lingard can help on defense while Martial can focus on attacking.
That is if we play 4231 but it could easily be 433 with Herrera replacing one of the attackers. In that case, it's more likely it will be Rashford and Mikhi on the wings, for their workrate. We could also go for the 343 with wingbacks, which complicates things even more.
We'll see what Mourinho is planning for Liverpool, but I doubt he will trust Martial to start games of this kind of importance just yet. Although, I'm curious of how he would do, I'm also afraid that if he fails badly, his confidence which he's gained so far during the season might take a huge knock. So I say it's better that we keep using him as we are now and gradually keep giving him more time to give him the chance to show he can do it when it matters.
Sorry for such a long post by the way.