However, Thompson stressed that the club are continuing negotiations with Blackburn Rovers with a view to bringing David Goodwillie back on loan.
The Scottish Cup-winning hero has endured a frustrating time since he left Tannadice for Ewood Park in August 2011, making only 10 starts, and United boss Jackie McNamara is keen to give him the opportunity of regular football at the club which launched his career.
Thompson said: “We are sorry to lose a player of Johnny’s calibre but trying to get David Goodwillie back is quite a move and shows our ambition to bring the right players in.
“We are also chasing further exciting additions to the squad. To help us do that, I would urge all our loyal fans to keep buying season tickets. They continue to offer fantastic savings compared to gate prices.”
Dundee United chairman ‘trying to get David Goodwillie back’