The Austria international, who signed a two-year deal at Bramall Lane in the summer following his release from Crystal Palace, sustained what is feared to be a serious knee injury in a midweek reserve-team fixture and is awaiting scan results.
The Blades have been hit by a series of long-term injuries to key players this season.
Club skipper Chris Morgan was ruled out for the rest of the season back in October when he damaged knee ligaments in the 1-0 home defeat to Coventry while striker Darius Henderson made a shock return to the starting line-up last week for the first time in 11 months following a hamstring injury - although he marked it by being sent off.
Other long-term absentees include Andy Taylor, Ryan France and goalkeeper Richard Wright.
Source: PA
Source: PA