Perez was our 6th forward option under EV behind Messi, Suarez, Griezmann, Dembele (until he got hurt) and Fati.
Now under Setien he would again be our 6th option after we'll bring Rodrigo (I know people don't like him but I think we'll get him). So Messi, Griezmann, Rodrigo, Fati and now it seems Collado will all be ahead of Perez.
He has played 70 total minutes in the league since September after we got Suarez and Messi back from their injuries. And Setien probably also saw enough of him in 60 minutes against Ibiza (though it's unfair to judge him based on this game when the whole team was bad) and more importantly in trainings to see that he likes Collado more.