Duff arrives from Shamrock Rovers, where the 39-year-old was part of the first-team coaching staff.

McManus, 36, joined Motherwell's coaching staff when he retired in 2017.

Celtic manager Brendan Rodgers said they are "real high-calibre individuals who have a huge knowledge of the game".

Duff, who played for Chelsea, Newcastle United, Fulham and Blackburn Rovers, will team up with Tommy McIntyre in managing Celtic's reserve team.

He told Celtic's website: "It is a massive honour to be given this opportunity and I am looking forward to playing my part in developing our young players of the future."

Former Scotland centre-half McManus began his playing career with Celtic and joined Well in 2013 after a spell with Middlesbrough.

He added: "It was a real dream come true to reach the Celtic first team and now it will be an absolute privilege to work every day with our young players to see if they can live their own dreams."