The veteran centre-forward damaged a shoulder during Tuesday's win over Plymouth and went off at half time.
Leeds' worst fears were confirmed when scans showed the injury is as serious as first thought and the 36-year-old will be out of action for at least six weeks.
The loss of Deane will come as a blow to Kevin Blackwell as the striker is United's top scorer this season with seven goals.
The Whites may now decide to move in January's transfer window to cover Deane's absence.