Sunderland forward Romaine Mundle and midfielder Alan Browne are both facing up to eight weeks out through injury.
The duo both had to be substituted in the 2-2 draw at home to Coventry City before the international break.
Mundle, who has been involved in all 15 of the Black Cats' Championship games, sustained a hamstring injury while Republic of Ireland international Browne, who has made six league starts, has suffered a fracture in his leg.
The Championship leaders travel to Millwall on Saturday looking for a first win in four after drawing their previous three matches.