Patrick van Aanholt is under assessment to ascertain the extent of his shoulder injury, which he suffered at Selhurst Park during Monday night's victory over Crystal Palace.
The Dutchman dislocated his left shoulder and was forced off just after the half-hour mark having made a significant impact in the game by laying on a cross for Steven Fletcher to head home the opening goal of the evening.
24-year-old van Aanholt was taken to a London hospital to receive treatment and they subsequently put the shoulder back into place before allowing him to travel back to the north-east with his team-mates.
Early projections suggest the former Chelsea full-back faces between four to six weeks out of action, but with a concluding prognosis yet to be revealed.
It'll come as a heavy blow to Sunderland boss Gus Poyet, who is now down to the bare bones in terms of numbers available to him in the defensive section of his squad.
Billy Jones is still out injured with a hamstring problem, which sees Santi Vergini covering in that position and new French signing Anthony Reveillere now filling in at left-back.
That leaves Sunderland's defensive resources stretched to the limit and with no clear end in sight with regards to Jones' availability.
There could be a place on the bench against Everton on Sunday afternoon for youngster Thomas Robson, who is equally adept on the left of defence and midfield.
Meanwhile, there was some better news in the shape of Jack Rodwell's return to training today after a knock to his calf put paid to any hope of him being involved at Palace.