probably all agree utd need a new #9 with good scoring record and not so injury prone as martial BUT the question is what type of #9 United or ETH's system want. a typical #9 like ztalan, cavani, lewandowski, haaland, nunez, etc or someone more fluid and mobile which is not a typical focal point ?
just look at city, last season they played without a focal point so they bought haaland. so far haaland has been banging goals and people are singing praises of him. if you look at the game vs liverpool, haaland was totally shutout as liverpool managed to cut the supply line to him. the is the same as nunez so far. liverpool before nunez doesn't have a typical #9, firmino is the more fluid and mobile type then a typical focal point and their front 3 of mane, salah and firmino were banging in goals like nobody business.
now if utd going to stick to 4-2-3-1 on the long term, a typical #9 like haaland type makes sense. however, if ETH intention is eventually 4-3-3 then a fluid and mobile type like martial is more suitable. this is probably the reason gakpo always come into the news as a potential new #9. he is more of the fluid and mobile type compared to the typical focal point like haaland