That wouldn't be a problem for this game - Burnley 3 Watford 0 on 15th December, when Watford couldn't raise a team. There would have been no rearrangement. The idea that Watford should have the game put off when they were short staffed, but Burnley should have to play now that Watford are back up to strength - hardly fair, is it.
And that is the issue now we are seeing games postponed for non covid reasons.