Watson was included in the club's first team squad whilst still a youth signing, being an unused substitute for United's last two matches of the 2005–06 season. After signing his first professional contract in December 2006, Watson's debut for the Dundee United first team came in the Scottish Cup third round match against St Mirren in January 2007, when he played 85 minutes in a 3–2 victory. Following this victory, he was described by his then manager Craig Levein as "without fear of contradiction, the best young player I have ever worked with".