The Northern Irishman secured back-to-back trebles in his first two campaigns at Parkhead and laid the groundwork for a third before he departed for Leicester in February this year.
"I've said it many times, it was incredibly difficult," he told the Daily Record. "It was a really tough call from a personal perspective and a professional one because my relationship with everyone at the club was great from the boardroom to the dressing room.
"But perhaps now people will have a better understanding of why I made the choice to go when I did. I had a lot to think about at the time. The players were absolutely phenomenal for me in my time there. We were on this great journey together. We created a culture which I loved coming into every day. And we were winners.
"We won - we were relentless at it and relentless in our work - and we had success together. So for me to then make the decision to leave them behind was not an easy thing to do. But I felt I had taken it as far as I could in that time. I then had a decision to make about going into a new project at Leicester where there is an ambition there and there are challenges that I would be excited by."