The Hornets were denied a fourth consecutive victory by Bakary Sako's late equaliser but Zola claims that his side should have put the result beyond doubt sooner in the contest.
"I was already thinking the game was won and the late goal was a shock for me as it was for the whole team," the Italian told reporters.
"I think the difference wasn't the goal we conceded but the other goal we didn't score. We had so many opportunities to do that and we couldn't quite convert them.
"It's the price you pay in those situations. When you play away from against a good team like they are, you can expect that sooner or late they are going to score a goal - even if it's a fortunate one."
Watford are now seven points behind leaders Cardiff City in the Championship.