Doak came through the ranks at Parkhead, made his first-team debut as a sub against Dundee United last January, and had a cameo appearance against Rangers a month later.
Hoops boss Ange Postecoglou was a big fan and had hoped to hold on to the talented kid, but the youngster opted to move to Merseyside for a fee of around £600,000.
Despite leaving the Premiership champions, Doak will forever be grateful for what they did for him.
He added: “At first, the decision to leave wasn't an easy one. But once I went down and saw the standards and the facilities, it's top level.
“At the same time, Celtic is the club who made me the person and player I am. I owe them everything.
“I still speak to a lot of the boys and keep up with the results."