Mackay was disappointed not to see the game go ahead but is switching his attention to Saturday's visit to Scunthorpe.
"I had a look myself and there were bits of the park that were not very good at all. It was the right decision," Mackay told the Hornets' official website.
"It's very disappointing for us as it's been so stop-start in the last couple of months in terms of the weather and the FA Cup fixtures.
"But we'll be back into work on Wednesday on the training pitch to focus on Saturday."