JON-PAUL Pittman is expected to be out for at least three months after having surgery yesterday.
The Oxford United striker suffered a dislocated shoulder during a game at Accrington Stanley earlier this month.
It was initially thought the 26-year-old would not need to go under the knife, with the club hoping he would return to action next month.
But after the shoulder "popped out" once again, an operation was needed, meaning at least 12 weeks on the sidelines.
U's boss Chris Wilder said: "When something gets dislocated, until the healing process is completed there's always a chance it might happen (again).
"It could be the most innocuous of things, like putting your shirt on, and it can pop out again.
"It's a big blow."