But there will be some new arrivals at the South Yorkshire club, with a new striker - understood to be Irishman Georgie Kelly - and a loanee coming through the door before Monday’s transfer deadline.
Sadlier leaves having joined the Millers from Doncaster in the summer of 2020, but boss Paul Warne’s switch in formation shortly after left him out of the picture.
Warne said: “It's just about the game-time. He's at an age where he needs to play.
“He's been in my office probably six times this season telling me how much he wants to play. His attitude has been spot on. He's always been patient.
“He was always aware of Bolton's interest in him. It was something he wanted to do, it wasn't something I wanted to do.
“The longer the season went on and the less game-time he got, the more frustrated he became. I don't want a hostage here.
“He's probably just outgrown his time here. Had I kept him beyond the window and then still played him one game in six, would I have had the same Kieran?
“I don't think I would have. I'm not saying he would have downed tools, I just don't if he'd have had the full level of motivation we need to get promoted.”
On the possible two new arrivals, Warne said they were both close.
“The loan player I want to get in is close I think,” he added.
““It is just the last little bit of finance and making it work for both parties. Through the parent club's permission I spoke to the player, I was as charming as ever so that's not a problem.
“The player wants to come, the club are happy for him to come now.
“I am confident of getting that player in.
“It's the best kept secret I am trying to get another striker in which I feel is very close.
“We haven't dotted the I's and crossed the T's. You can guess who and there is a good chance your guess might be correct.”