Celtic are now 13 points behind Rangers in the Scottish Premiership table, meaning even if they win their two games in hand they still face a considerable gap to overcome.
The Celtic board have continued to back Neil Lennon and they will only review the situation in the new year, as they look to chase down Rangers at the top of the table.
McManus believes the Celtic board should have acted when they lost to an under-strength Sparta Prague side a home in the Europa League and they could have kept Rangers within their reach in the title race.
He feels the supporters have every right to feel aggrieved at the board for their approach, but warned it is now too late to expect any new manager to catch up with Rangers.
McManus said on PLZ Soccer: “I said a month ago that the board had got a serious decision to make if they wanted to contemplate changing the manager.
“Fast forward a month, they are out of two tournaments and I think the league is gone after they dropped another four points.
“The supporters have every right to be really angry at the board for not making a decision.
“I think we could all see after the Sparta Prague game, where Celtic lost 4-1.
“It was a humiliating defeat against a team that were plenty of the players missing.
“I think after the game there must have been a serious thought of changing the manager and if they had changed the manager back then, maybe they would still be in the League Cup and maybe still closer to Rangers.
“Whoever they bring in now will not be a miracle man.
“The league is gone for me barring an unbelievable collapse from Rangers from here.”
Celtic have won just two of their last 13 games in all competitions, but will look to get back to winning ways on Thursday against Lille.
