What would you do when the player is not being honest with you? Amorim asked him if he went out before the Everton game, he said he didn't. Later again he apparently appeared fatigued in training and it seems he out the day before training again. Pretty sure they have tried everything they can to deal with this internally. He had previous episodes of being unprofessional with ETH too, remember the Belfast nightout where he was out until 3 am in the morning and then called in sick for training? Or oversleeping to the point where you were late to a team meeting? Or the team leaks.
Now under Amorim it appeared to be the last straw and he all along said Rashford will be back in the team if he changed his ways. That comment about picking his coach was made after weeks of being left out at which point it appeared as a last resort sort of comment from Amorim to help Rashford change. He dropped Garnacho at the same time and he was back in the team very soon. If anything Rashford has shown clearly he doesn't care about improving or helping his boyhood club. In his eyes, Rashford thinks he does nothing wrong even after years of underperformance and laziness on the pitch.
https://www.nytimes.com/athletic/6035672/2025/02/01/marcus-rashford-manchester-united-missing/