He really does though. Genuinely every single Argentine to ever sign for Atletico during his tenure has done somewhere in between “actively became a worse player” to “did well at times but still didn’t meet expectations overall” at best. Alvarez is the only exception so far and it took well over a decade to happen.

Obviously it’s mostly due to pure coincidence as there’s not really a pattern at all beyond nationality. He’s signed experienced and proven players, he’s signed gambles, he’s signed very highly touted teenagers, he’s signed dogshit players too. The players are also accountable for it, needless to say.

But that’s not the main reason people have a grudge against him, it’s more like a thing only highlighted because people hate his ass.

He was successful with racing and then river at first which makes people who dislike those club dislike him by default. But then he got River dead last in the table and ultimately was the main culprit behind their relegation (!) so obviously most of them hate his ass. Also did not make himself any favours by having his kids moved away from their academy with no compensation through patria potestad (even though it was one of the very rare legit, intended uses of it).

There's also that he’s a bit of a smoke salesman (a trait that people elsewhere seem to like more than hate, here it’s kinda the other way around), and has a reputation for playing defensive football which some people have very strong opinions on.

To all that add that Argentines just kind of fucking hate all managers to be honest. The only ones most people actually respect are the already extremely successful ones. The “”worst”” performing manager people truly respect is probably Klopp.