Former Coventry City loanee Patrick van Aanholt is out for up to three months.
The Dutchman, who is on loan at Sunderland, suffered a dislocated shoulder in the Mackems 3-1 win at Crystal Palace and Sunderland boss Guy Poyet has told BBC Sport: "Unfortunately Patrick is going to need an operation which means he is going to be out for two-and-a-half to three months," Poyet added.
"It is a big loss, but we will help him and we will have to cope with it."