It was widely reported back then. I googled it and found articles like this : https://www.foxsports.com/stories/nba/pacers-move-on-from-mcmillan If I remember correctly he had problems with Dipo, Brogdon and Turner I think, so it wasn't just one player. Now who is at fault is another story but when a coach has lost the lockeroom since you can't fire all the players you have to try to fire the coach.
That info is being based off Ric Bucher’s sources, and that article even cites sources from Chris Broussard. I don’t take either one of them seriously.
