The Spartak Moscow winger, who is preparing for the Champions League clash between the two sides at the Luzhniki Stadium tonight, left Parkhead to join the Russians in August 2010 but admits he is keen to again play for the Hoops.
"When I left I always thought it would be good one day to go back to Celtic. Whether that's now, next year or at the end of my career no one knows," said the 26-year old Republic of Ireland international.
"That was the one thing (Celtic chief) Peter Lawwell said to me when I left. He said you never know what can happen in football, so maybe the door's still open.
"Of course I miss Glasgow, but you can't stay at home forever. It was the place I grew up and I played for Celtic until I was 24.
"My friends and family are there so it's difficult to be away at times.
"But part of growing up is moving away and experiencing a new culture and that's exactly what I've done here.
"The past few years have been a big learning curve for me, not just in football, but I can't say I have any regrets.
"I still think about the big games I played for Celtic. They are memories that will live with me forever."