Brown has decided to move on from Celtic after spending 14 years at Parkhead and winning 23 major trophies with the club.
He has agreed on a pre-contract with Aberdeen and will be joining the Dons in the summer in the capacity of a player as well as a coach.
McManus feels Brown did not get any assurances over the identity of the next Celtic manager from chief executive Peter Lawwell before deciding to leave the club in the summer.
He believes Lawwell does not want to lose the Celtic captain, but since the club are yet to identify a new manager, he could not give any certainty to the midfielder.
McManus said on PLZ Soccer: “Peter Lawwell and Scott Brown are very, very, very close.
“I think Scott Brown wanted to know who the manager was going to be and Peter Lawwell either didn’t know or didn’t want to tell him.
“I would think he didn’t know.
“I don’t think he would have let Scott Brown go without telling him who the manager was going to be because he wanted to keep Scott Brown.
“I don’t think Peter Lawwell, over the last couple of weeks, knows who the manager is going to be.
“I don’t think it’s done, there might be people in the background being spoken to, but I don’t think they know who the manager is going to be specifically.”
Celtic are on the lookout for a new manager and have been heavily linked with a number of names, including Roy Keane.
