The 30-year-old has not given up hope of continuing his career in the future, however.
McManus received a second head knock of the 2015 season in August last year, and it was feared that he would have to retire from the game.
The Scotland-born winger has also been working with the Knights’ commercial team in recent times.
“I need rest and time to work through my options,” he said in a statement.
“This has been a difficult time and I certainly appreciate everyone’s support.”