It is deniable. Neither you or me know whose decision that was.
Prior to this Postecoglou has been very careful with player welfare. He subbed Romero off against Brentford for concssion concerns in his 1st ever game. He's taken a lot of time to bring players back carefully. Maybe this all changed with the pressure he was under
Or maybe it was just terrible luck.
We don't actually know. If the players didn't think they were ready they would have leaked it (or their agents would)
Look if you want to believe Ange has done a great job of taking care of our players health with his tactics, rotation, and roster choices, you're free to believe what you like.
For me the circumstantial evidence about the whole situation suggests he's just not capable of maximizing squad effectiveness given the resources at hand and constraints in place.