O’Neill said the former Manchester United defender would be sidelined for the next eight weeks due to the ligament damage sustained during last weekend’s FA Cup tie against Middlesbrough.
“We have a few defensive problems now with Wes being out for a fairly lengthy time,” O’Neill told reporters at his post-match press conference.
“News coming through today might not be as bad as I feared but it is still going to be a long time. I thought it was important for us to try to get a few in.
“I don’t think Wes needs an operation but he will have two weeks in a brace, two weeks elsewhere and you are probably looked about eight weeks before he is back,”
“He came down on his knee and it just cranked up. We had thought that at one stage that it looked as though it would be a season-ending injury.
Following Brown’s injury, O’Neill signed former Liverpool defender Sotirios Kyrgiakos on loan for the remainder of the season from Wolfsburg and Wayne Bridge, who has been loaned to Sunderland by Manchester City.
Bridge made his Sunderland debut as a substitute in the resounding defeat of Norwich which saw Fraizer Campbell score a superb long-range effort before setting up Stephane Sessegnon’s sixth goal this season and then seeing Daniel Ayala score an own goal.
O’Neill reserved special praise for Campbell who was making his first start since August 2010 due to knee problems.
“I am delighted for Fraizer Campbell. You should just see him in the dressing room with that big smile of his,” O’Neill said.
“But it is more than that. It is more than just his enthusiasm. I thought he played splendidly for us tonight. He got a great goal and had big say in the second one too with a great cross.
“His overall attitude is terrific and his overall performance was brilliant. I think that he has surprised everyone. Maybe I shouldn’t be surprised because the medical staff have been heaping praise on him for his attitude throughout the time he has been injured.
“There were a lot of dark days there because 18 months is a long, long time to be out of the game, especially when you are young.”
O’Neill, whose side are now eighth in the Premier League, continued: “He has given us a big lift at an important time.
“I was really pleased with the performance. We played well, we ran with it, we passed it well; we showed a lot of enthusiasm and got plenty of players into the penalty box, particularly on breaks in the second half. There were three, four and five Sunderland shirts getting in there.”