The Dons manager told BBC Scotland: "It's a club on the up. It's a good level of football and they are a very ambitious club. I'll not be churlish, I'm sure he will flourish. Unfortunately he is not in the red jersey here.
"My reaction is disappointment that we are losing him. He seemed anxious to go and he was advised that his future was elsewhere.
"In spite of the fact that I would have liked to have had him here and that it might have been better for his development here, he has elected to go there and there is not anything you can do about it really."
The midfielder made his debut for the Scottish Premier League side in 2010.