Under new boss David Moyes, United's previously impregnable fortress has already been breached by visiting sides on four occasions so far this season.
But Williams does not believe a trip to the Premier League champions has got any easier as Swansea look to end a run of seven games without a win in all competitions.
"It's an important game as we haven't won in so long, but going to Old Trafford is a special occasion for any footballer and we will go there," said the Wales defender.
"We will try and enjoy it and we will try and put a few things right.
"I think they are a top class side, there are no worries about United as far as I am concerned.
"With the transition with the new manager things can take time and it was going to be tough. But they still have a lot of the same players from last season and they can play a number of their players in their squad who are world class."
Swansea face United with a host of injury problems after Pablo Hernandez's hamstring injury saw the Spaniard join Michu, Michel Vorm and Nathan Dyer on the casualty list.
United stopper David De Gea is confident they will succeed at Old Trafford.
"The FA Cup is a competition with a lot of history,'' he said.
"There is a lot of romance associated with it and it would be terrific to win it.
"Being at home is a boost because we will have so many fans behind us.
"Still, Swansea are a tough side and it will be a complicated game for us.''
- Sports & Recreation
- Manchester United
- Premier League
- David Moyes
- Old Trafford