The third-round meeting, postponed twice due to a waterlogged pitch, is due to finally go ahead after The Valley passed an inspection yesterday afternoon.
It will be assessed again when referee Stephen Martin arrives this afternoon, but United are preparing for the game to go ahead as planned.
The delays have caused frustration in both camps, but there is a belief it has improved the visitors’ odds of claiming a scalp.
After a two-week break, the U’s returned to action with a goalless draw against Portsmouth on Saturday, but Charlton have not played since New Year’s Day.
Mullins is looking to exploit both their lack of action and the state of the pitch.
He said: “I think with Charlton not playing for a while, hopefully we can take advantage of their possible rustiness.
“It does take you a bit of time to get going. You can do as much training as you want and play practice games, but nothing really prepares you for a competitive game.
“Hopefully we can catch them cold.
“Another one is the pitch, which is probably not in the best of condition.
“I know Charlton like to get the ball down and pass it around.”
A visit to Huddersfield in the fourth round on January 25 awaits the winners.
While United have not reached the last 32 since the 1998-99 season, it would not be uncharted territory for one of their players.
David Hunt was part of the Crawley side, then in non-League, who went all the way to the fifth round, where the run was only halted by a 1-0 defeat at Manchester United.
The full back knows exactly what is required to stage an upset and thinks United have what it takes.
“You always have the belief,” he said. “That is one of the joys of the FA Cup – there are always upsets.
“(Crawley) were in a good run of form and we had a lot of luck. We had a good dressing room, which is very similar to here.”
Victory for United would improve what is becoming a lucrative run in the competition. They have already netted £150,000 in prize-money and television revenue so far and tonight’s winners will receive another £67,500.