Ruben Amorim claims Marcus Rashford failed to adapt to his training and matchday demands at Manchester United.
Rashford, 27, did not play in 12 successive matches for United before he joined Aston Villa on loan for the remainder of the season on Sunday.
Villa are covering at least 75% of Rashford's £325,000-a-week salary and have an option to buy the England international in the summer. United are expected to move for a proven goalscorer in the next transfer window.
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Rashford didn't play for United after he was substituted in the 2-1 Europa League win against Viktoria Plzen on December 12. The England international did not acknowledge Amorim when he was taken off in the second half with United 1-0 down.
Three days later, Rashford and Alejandro Garnacho were omitted from the squad for the Manchester derby victory at City on account of their attitude. Garnacho has since appeared in every United match but Rashford outlined his desire for a "new challenge" two days after he was left out of the squad against City.
Amorim said last week he would not be "embarrassed" or "humiliated" if Rashford regains form at Villa, who have qualified for the Champions League round-of-16 automatically. On Thursday, the United head coach elaborated on why Rashford was let go.
"That is of course, you as a journalist or a fan, can take this in the way that you want," Amorim said. "You can take whatever you want.
"What I say is that I couldn’t [get] Marcus to see the way you’re supposed to play football and to train the way I see it. And sometimes you have one player who is really good with one coach and the same player with another coach is different. I just wish the best to Rashford and Unai Emery, and they can connect because he’s a very good player.
"Let’s focus on the case, the case was some of the guys don’t adapt the way I see football, the other guys adapt, and we are trying to build the team."
When asked if Rashford had openly expressed his reservations with Amorim's style, the Portuguese added: "It’s not the way that occurs, it’s something that you feel, as a coach and a player. It’s quite normal. It happened with a lot of coaches.
"The important thing is that I’m here saying it was my decision. And Ty (Malacia) and Antony, it was my decision to do these loans and to keep some players even without any transfers. I am just here saying that was my decision."
Luke Shaw has not started for United since February 18 last year and played only three times since, having sustained calf and hamstring injuries this term.
The 29-year-old has now sustained a separate setback and Amorim is unsure when he will be availble to play again. "It’s hard to give a time because he has a small issue, different from the injury. Then he has to stop, then we have to return a little bit, we have to work the fitness, to be aware of the last injury.
"It’s a complicated moment to tell you a timeline. I don’t want to put any pressure on the medical department, on Luke. We just need the players fit to help the team."
