I don't understand what Martial has done to become irreplaceable. Martial has averaged a goal every 215 minutes of PL competition and a goal every 424 minutes of CL competition at United. Haaland has averaged a goal every 47 minutes of CL competition for Salzberg. Just because Martial is the best we have right now shouldn't make him irreplaceable.
Haaland would be cup tied at Dortmund, Leipzig, or Juve, so less playing opportunities at those clubs. He's good enough to challenge Martial for the starting #9 spot. His combination of physical abilities and technical skills give us something we don't currently have against teams that sit deep. Just put a little more money in his and his agent's pocket than the other 3 suitors are offering and he's ours. Why not?
It's absolutely nothing to do with Martial.
Bringing Martial in to the equation as to why Manchester United as a club and Ole as a manager may not be able to or want to bring Haaland here is not worthy.
It could be to do with money, it could be that Ole isn't being backed or we can't afford him or his agent fees, maybe Ole knows him well enough to know he may struggle or we'll enough to go after him but the player not being interested in moving here for so many reasons - CL, previous problems, his own fanatics.
I don't know why bringing Martial in to this is the reason we supposedly don't go or don't get Haaland? What proof is that? Ole might even be thinking that Greenwood is better than Haaland after coaching then both & wants to give him more time.
I'm sure if Ole was interested in Haaland and thought he was good enough for United after previously managing him, if he was allowed a transfer window to spend money freely in positions he thought were needed all topped out with Haaland wanting to move here - then the likelyhood of it happening looks extremely high and Martial, Rashford or any other player has hardly anything to do with it.