These sort of situations, where teams spend a ludicrous amount of money on a player & find out he is total shit, usually end in a loan plus option to buy deal.
That's not the situation Atletico Madrid are in with Felix though.
They've been pretty happy with how he's played, but he had a few injuries and it was AM's worst season in a long time. The problem he had is that Simeone has very different demands to Benfica, he had to grow up a lot - he'd never left home before he became a €126m player - and had to put on weight - he looked like a 15 year old 12 months ago, braces and all.
He also got let down massively by Morata and others not finishing chances he put on a plate.