Boss Alex Smith reckons the former Cyrstal Palace midfielder will not kick a ball for the rest of the season after dislocating and breaking his right ankle in two places against Kilmarnock in an incident which stunned Tannadice.
It took paramedics seven minutes before they could stretcher Fullarton off the pitch on Tuesday night and into a waiting ambulance because they had to administer oxygen to him as he clutched his leg in shock after a tackle with James Fowler.
"He will be out for at least four months," said stunned Smith. "And when you take into account the rehabilitation this injury will need, he will not be ready until the end of the season."
Fullarton, who was only signed by Smith three weeks ago, has a contract with United until the end of the season, but Smith said: "We will worry about that later. The most important thing is that he recovers from this."
But the Tannadice manager admits that Fullarton's sickening injury has deprived United of a real presence in midfield.
"It is a big blow because Jamie was going to be an influential person for us. He supplied balance on the left side.
"Now I may need to see if I can get someone in on loan because we have not used up our loan options yet, and maybe ask around clubs in England."
The 1-0 defeat inflicted by Craig Dargo's late goal left United bottom, but Smith said: "The players were affected by what happened to Jamie and turned off their concentration after the seven minute break."
