Sunderland winger Ross Wallace will miss the season after damaging knee ligaments in training.
Wallace was one of Keane's first signings
The 22-year-old has made 21 appearances this season and his absence will deal boss Roy Keane a blow as the club battle for Premier League survival.
Keane may now step up his efforts in the transfer market to bolster his squad before the end of January.
He is keen to lure Chelsea midfielder Steve Sidwell on loan and has been linked with Birmingham's Olivier Kapo.