The Scotland international winger declared: "I've no regrets about leaving Celtic.
"The most important thing was getting away to play regular football. I've managed to do that with Derby and everything that's followed has been a big bonus.
"It's great to be involved in the international setup again because it had been a while since I was in a full squad. I couldn't really think about Scotland when I was in that situation at Celtic. I had a job trying to get into their first team squad, but it wasn't to be. It was up to me to make the decision to move on and it's worked out well."