The centre-back was sent back to parent club Brighton & Hove Albion after it was discovered he would miss the final two games of Celtic’s disastrous season through injury.
The 29-year-old hasn’t played since February after struggling for form all season. A Republic of Ireland international with over 100 games of English Premier League experience, he was expected to be the final piece of the puzzle as Celtic looked to secure ten-in-a-row, but he soon became the poster boy for the club’s dismal performance.
In his post on Instagram, Duffy hinted that a “tough” year off the field may have affected his impact on it.
He wrote: "Grateful for the opportunity to get the chance to play for a club that many dream about doing.
“Thanks to the supporters and the players for helping me through a tough year on and off the field. Not many know what goes on behind the scenes.
“Time to hit restart button and get myself sorted. Celtic football club will always have a place in my heart, hail hail.”
Duffy lost his father in May 2020 at the age of just 53. In an Instagram post at the time, the player wrote: “You weren't just my dad, you were my hero, my idol. I'll missing ringing you after the match and you telling me how s***e I was.
"Every time I put that green shirt on for our country I'll be thinking about you because I know how proud you were of me playing for Ireland."