He joined the Scottish Premier League club in April, making seven competitive appearances without scoring a goal.
He had not played for 18 months after being convicted of causing death by dangerous driving after an accident on the M1, where he lost control while trying to change the music on his MP3 player. He was given a 32 month jail sentence in October 2009.
Robertson came through the tanks at Sheffield United, but did not make a first-team appearance for the Blades. He had loan spells at Torquay, Northampton, Dundee United, Oldham, Southampton, Ferencvaros and Bury.