The versatile defender suffered a knock against Groningen during the Black Cats’ 3-2 win.
Brown is due to undergo scans on his knee when the club return to England, and O'Neill is hopeful that the former Manchester United man will not be seriously injured.
He told reporters: "He will be assessed. He is doing some scans and I'm hoping that the news is going to be okay. But his knee is really swollen."
Prior to Brown picking up the injury Sunderland had given Norwich permission to talk to defender Michael Turner over a potential move to the East Anglian club and O'Neill admitted he was happy to let the former Hull City man leave due to his options at the back.
He continued: "Norwich had asked us about it before Wes' injury and so having given permission for Michael to go down there and the clubs having agreed a fee, it was just ironic that Wes got injured at the time.
"We will see what develops down in Norwich, but I am hoping that Wes' injury is not as bad as it might have been.
"But we do have a number of centre-halves at the football club at the moment, and priorities in strengthening are probably elsewhere."The 60-year-old revealed that he is chasing targets to add to his squad at the Stadium of Light but he is unconcerned at a lack of deals so far.
"There hasn't been a phenomenal amount of movement in the transfer market," he added. "That doesn't really matter, I'm not too concerned with what other clubs are doing.
"We have an eye on one or two players and we are hoping to be able to do something. We all know what's needed at the football club and I will do my very best to ensure that happens.
"We have a number of things that we are moving on at the moment. They tend to drag on a little bit, that seems to be the nature of things.
"But I am hoping we can resolve one or two of those situations in the not-too-distant future.”
O'Neill was also asked if there was a possibility of the Black Cats making a sensational move for former Newcastle striker, Andy Carroll.
He smiled: "Andy Carroll is a really good player, a really good player. Would he come to Sunderland? He is a very fine player."