What's there to be baffled at? He's an elite CF prospect. Top CF prospects are very rare. Bayern are signing a 30 year old after they went a year without a striker because their mid 30s striker left to go to Barca. Madrid lost their mid 30s striker and are using a striker less formation now, waiting to sign Mbappe who is a wide forward anyway. City have Haaland who along with Osimhen are literally the only under 30 year old strikers who are ready for top teams. Before getting Haaland, City went a year without a striker too. Liverpool don't really have a striker, just a bunch of wide forwards. Arsenal have Jesus who is a CF and works hard but doesn't score much.
The CF market is shit is what it is. Football stopped developing strikers so when you get the chance to sign a top prospect, you simply pay up if you want him.
You have Haaland, Kane, Lewandowski, Osimhen, Benzema who are the top level strikers currently. 3 of them are 30+, 1 is at City and the other is getting quoted at 150m+, and was bought by Napoli before he did much for 70m anyway.
You have a bunch of wide forwards who teams try to adapt to make do at CF, to varying degrees, like Jota/Gakpo, Isak, Kolo Muani, even Mbappe, who aren't proper #9's and don't really do that hold up play and being a focal point (or to varying levels).
You have mid tier players like Lautaro, Vlahović, Toney, Ramos, Watkins, Mitrovic who are being quoted absurd fees without having top level potential, so you would pay elite level prices for Europa League production out of them as their ceiling.
Then you have a few youngsters with big potential, like Hojlund and Sesko. And of them, Hojlund is the one of the very few on this entire list who is a proper #9 with all the tools to be a top center forward. Think of it like paying to get a CF in the mold of Dzeko, Lewandowski, Ibra, Kane, Drogba, Cavani, etc at the age of 20. None of these guys did anything at that age, most broke out at 22/23 (Kane at 21). The second these guys break through, they are over 100m and pretty much unobtainable if you didn't get them beforehand (unless you are Haaland who puts a clause in). It's securing a rare profile, elite talent that we happen to be in dire need of as well, and a player who may not be all that far away from stepping up either.
As for who starts... Martial doesn't have the pace or strength or movement in the box that Hojlund has, despite the age. Yeah Martial will get his gametime when fit which is good. Hojlund is barely 20. By the time Martials contract is out, Hojlund might just be ready to be the full time guy. He gives us that threat in behind that only Rashford offers in our squad, he gives us proper #9 movement in the box, he gives us a focal point.
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