The Hornets were 1-0 ahead at Cardiff City before red cards to David Pudil and Nathaniel Chalobah allowed Cardiff to sneak a 2-1 victory in the final minute of the match, and Zola has admitted that being reduced to nine men changed the game.
Zola said: "It's very cruel, not just for me but for the players because of the way they played and performed.
"To see them come away with a defeat is hard to take. I feel sick for the players.
"At 11 versus 11, we were 1-0 up and in control. The balance of the game changed completely with the red cards."
Watford are currently in 15th place in the Championship table.