Disagree massively.
His hold up and bringing players in is decent enough and having someone like Bruno, Eriksen and Antony (similar to what kharva was doing today, cutting in and whipping a cross in) playing crosses into.the box like today would give us so many more goals than we currently have. We have no one who occupies or hits the box, Rashford still proves that's not his forte and Martial is no different.
How many headed goals does Haaland have for City and how many headed goals does Osimhen have for Napoli in their respective leagues, start there.
Then understand that Rashford isn't going to become a balanced wide forward like Grealish or Mahrez for City, or Martinelli or Saka for Arsenal or even Son. Marcus is a goal scorer plain and simple now. He's getting into positions entering the box. If Martial was fit, he'd make the difference between easily in 3rd place if not at least 2nd. Martial would easily be around 8 to 10 goals in the league by now.
But there is a clear difference between how Martial plays and how the team plays, compared to Osimhen plays and how he plays at Napoli.
I'm not disputing that Osimhen is a very good goal scorer, but what I am correctly questioning is how he would fit within the United squad and how that meshes with Marcus and Antony, assuming they are first choice wide players with Garnacho, Sancho, and I guess you can throw Pellestri and Amad in there too. Those players don't spam crosses and hope you can make a good run and good headed attempt for a goal.
While they have the ability to do so (crosses) and have done so, it's not their first thought of head up and cross into space, just not their thing. They all make contributions going into the box and looking for the runs. Making the extra pass for a better opportunity.
Unless United get strong technical players out wide who can cross a ball and pass in general from anywhere into space and have a Haaland to aim for, Osimhen will require even more adjustment and compromise to how they have been setup to play by EtH.
That is why keeping Martial (fit and available) is important and having a reliable starter health wise who is an asset for the team across multiple phases is key.
I'd rather have a Wayne Rooney type player come into CF than Osimhen. That's a Muani, Benzema, even Alvarez at City, Watkins at Villa, Kane...those would be more than sufficient and possibly with the exception of Kane, 2/3rds the cost plus much better continuity of play than Osimhen.