A pitch inspection has been set up for 11am at The Valley amid concerns the third-round tie could be thwarted by a waterlogged pitch.
Constantine Hatzidakis, a Football League assistant referee who is affiliated to the Kent FA and lives locally, could postpone the game more than 24 hours before kick-off if conditions are bad enough.
The playing surface was heavy in Charlton’s games during the Christmas period and further rain was forecast for this morning and for most of Saturday.
A postponement would be hugely frustrating for United, whose New Year’s Day clash at Newport County was washed out.
That game will have to be rearranged for a Tuesday night later in the season, along with the trip to Bury, which was scheduled for tomorrow and had to be cancelled due to United’s involvement in the FA Cup.
The squad have also experienced it earlier in their cup run, when the first-round replay at Gateshead was called off just before kick-off.
Deane Smalley does not want to lose another game to the weather and the striker is wary about it leading to fixture congestion.
He said: “We had it with Gateshead, although I wasn’t involved with the game that was called off.
“It wasn’t ideal having to go there again, so hopefully it will be on and we can get it out of the way.
“Otherwise there will be a build-up of fixtures, like there will be with the Newport game.”
United have already visited The Valley this season, losing 4-0 in the Capital One Cup first round last August.
Smalley does not believe the result will have any bearing on the meeting in the FA Cup and even thinks the heavy pitch could end up helping the chances of a shock.
He said: “We’ve got to go there and put it right, we obviously don’t want the same (outcome) again.
“It’s a different competition and there will be different personnel.
“I’ve heard the pitch isn’t the best, so that might suit us a little bit.
“They passed us off the park in the second half, so with a dodgy pitch it might even itself out a little bit.”