Assistant boss Glynn Snodin said: "Simon has a bit of sickness but he will be okay. He didn't want any of the players to catch the bug so he went home to bed."
The news that Grayson is likely to be in the dugout at The New Den will be a major boost for the United players ahead of what is certain to be a pulsating 90 minutes in south London.
United have no fresh injury worries as they look to avenge a 3-1 defeat at The New Den when the two sides met last October.