In what is a huge blow to the Arabs on the verge of the Scottish Premier League season, Robertson was injured in a pre-season friendly with Forfar and will now miss the start of the new campaign.
"Scott looks like he's going to be out for four to six weeks," United boss Peter Houston told the Daily Record.
"His ankle is really swollen. Scott is a fit boy and he works hard so he'll be back soon.
"It was unlucky as the Forfar player just collided with him when he was shooting. There's nothing you can do about these kind of injuries.
"However, I have been really pleased with the progress of some of the younger guys."