United have three players called up to U-21 squads and were not allowed to retain Scott Carson as goalkeeping cover for Tuesday night.
So an application to the Football League was made for the game to be postponed.
"I am extremely disappointed because once again we find ourselves a fixture behind," said Hornets boss Ray Lewington following the announcement. "We have planned for this game and we are disappointed to say the least.
"It seems strange. They asked us to move from the Monday to a Tuesday because the police were unhappy. Apparently there is a music festival going on over the weekend.
"But now, because it is on a Tuesday, they can invoke the four-day rule before Saturday's internationals.
"There is nothing we can do but I don't think any of the boys involved in the internationals are in the first team. They maybe on the bench."