Realistically speaking, we cannot play these 4 players together as starters.
The reason underneath is simple: those 4 are our best attacking players, and if we start them all together then the manager doesn't have any meaningful firepower on the bench.
For mourinho who is known as an expert tactician, this is most certainly a no go: he needs options on the bench in every department so that to react to the way the match goes.
That said, we definitely need 1 (just one) additional pacey attacking player in the squad, and then obviously the above mentioned formation becomes a possibility - but there needs to be a transfer first, someone good needs to be purchased.
To add to that: what I mean is not that Martial must be on the bench unless another attacker is bought, but rather that someone from the aforementioned front four must be on the bench (and not Lukaku from tactics perspective, so it's mostly the trio of Martial, Rashford and Miki for two spots in the squad)