The 33-year-old former England star suffered a knee injury on a pre-season tour of South Korea and has failed to complete his rehabilitation despite sporadic returns to training.
Black Cats manager Martin O'Neill told the club's official website: "It's been a long time out for Wes, who has been beset with injuries.
"He looked as if he was just settling at the time I came in, but hurt himself in January last year and we've not really seen him back since.
"He had a setback out in Korea but it looked like he was making progress, but for Wes it's been a case of two steps forward and three back.
"Chances are we probably won't see him between now and the end of the season, but perhaps it'll be one he wants to forget and start again next season.
"He's joined in training on a few days and then has to take some time out again. At the moment, however, I think he's reversing that trend - it's one little step at a time at the moment."
Brown arrived at the Stadium of Light in 2011 from Manchester United for an undisclosed fee, signing a four-year deal.