This was what I was saying.
I'm not protecting Martial and blaming Rashford and Greenwood but more looking at the front 3 and seeing what's up.
The 4231 cant play a false 9 as good as a 433 in my opinion, the inverted forwards are able to get more central in the latter whilst in the 4231 we see Greenwood particularly struggling to ever find himself in the box in comparison to the touchline most of the game.
Then you compare Martial he is the one playing deep still.
The 4231 is very similar to the 433 but it has flaws in my opinion especially when trying to play false 9's and inverted forwards.
I think in the 433, for example if Martial drops deeper usually you'd see the two no. 8's going ahead of the ball to occupy the centre backs, while the inside forwards make those inverted runs. In a 433 set up, those runners off the ball would be Bruno and Pogba probably, and it could arguably be effective.
In the 4231, Martial is dropping deep into space that Bruno already occupies, and often nobody is making runs centrally in behind, because we have two holding midfielders. Against Leeds, we saw the potential with McTominay breaking the line, but that was down to Leeds moreso than how we play.
Look at how Liverpool like to play when everyone is fit. Salah and Mane make those out to in runs. When Firmino drops, Henderson or Fabinho sit (whichever is playing) while Keita and Wijnaldum go ahead of the ball. You need runners in behind to create space for the forward dropping deep, and also so those inverted runs are effective, otherwise the centre backs are just going to go with Salah or Mane coming infield.
In short, as you've said, if we play with two holding midfielders, then we need a CF who will also threaten in behind. It's essential to a functioning 4231, otherwise you end up with your 4 attackers square outside the 18 yard box facing the entire defence (something we see too often).
Bruno does brilliant to get in behind when Martial drops deeper, and his numbers back up his willingness to occupy that CF role when Martial comes short. But we also need that threat in behind from our striker. We can't rely on Bruno and Rashford to deliver all of the time.