This is exactly the problem, not just with EtH signings, but our litany of awful signings for years now.
We don't have a proper scouting and recruitment setup. If we did, Ten Hag wouldn't just sign his old players or lads he's seen that might fit.
EtH might identify Antony, for example, as a signing he would like with attributes for his style of play. At that point our recruitment department should be saying "ok, well here's what we think he's worth. Here's a couple of alternatives if Ajax play hardball, and here's a couple of players that we, as a professional scouting and recruitment team think would be a better fit for a top club like United.
But what happens is, United just go. "Derr ok then, let's sign him!" And proceed to get rinsed for a non-United quality player.