The former Manchester United man sustained a knee injury during Sunday’s FA Cup draw with Middlesbrough.
O’Neill admitted he feared the 32-year-old would be forced to sit out the remainder of the season.
However, scans have revealed that the problem is not as serious as initially feared.
"We don't think he needs the operation but you are looking at probably about eight weeks before he is back," said O'Neill
"But, at one stage, it looked like a season-ending injury."
Brown is not expected to feature again for the resurgent Black Cats until April at the earliest.
By Ben McAleer
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.