Brown, who spent 14 seasons at Old Trafford, feels the current United side have lost a little of their fear factor after losing six games so far in the Premier League and being dumped out of the FA Cup at home to Swansea.
The 34-year-old is keen to pile more misery on David Moyes' side and feels now is the ideal time to take on United.
"Are they vulnerable? Yes because this is a semi-final and everything can happen," Brown told the Daily Mirror.
"We are at home first so we have got to get a result, and away we will also think we can get a result with United's form at home.
"So it is something we are looking forward to. We have to take advantage of an opportunity like this, although they are still a top side no matter what has happened recently. After all, they won the league only a few months ago.
"But United's inconsistency does give you confidence. They have lost a few games at home and going there as a visiting team, it is nice knowing that. It does give you that little confidence boost. In years before it has been the other way round.
"This cup will be 100 per cent a priority for United now. They will be up for it. If they got to the final it would be a real boost for them, but maybe we can stop that."
Boyhood United fan Brown is confident that Moyes will get things right at Old Trafford, admitting it will take time for him to replicate the success of Sir Alex Ferguson.
"There has been a big change, with Sir Alex Ferguson going and David Moyes coming in," added Brown. "It isn't clicking now but it will."
Watch Sunderland take on Manchester United live on Sky Sports 1 tonight, with coverage from 7.30pm.