Chris Wilder is relatively happy with the result of the scan Samir Carruthers underwent on Monday.
The midfielder is expected to be out of action for between two to four weeks and has already begun his rehabilitation.
Boss Wilder commented: "We are relatively happy with the outcome, he's strained ligaments and our medical team are managing the situation in-house. In the 14 minutes he played at Southend he gave everyone a glimpse of what he's about and the score moved from 2-1 to 4-1 when he was on the pitch."
The substitute appearance at Southend was the midfielder's debut after joining the Blades last week from MK Dons for an undisclosed fee.