Goals from Will Hughes and Andre Gray either side of an own goal from DeAndre Yedlin handed the Hornets all three points from their trip to the North-East, in the process condemning the Magpies to their fourth straight defeat in the league.
The result saw Benitez's side drop to 13th in the Premier League - just four points above the relegation zone - and speaking after the final whistle, the Spaniard said that his players will now have to 'work as hard as possible' to get moving back up the table.
"We made a couple of mistakes and we paid for that," he told BBC Sport. "They had a couple of runs down their left which were difficult to control. We were pushing in the first half, we were on top for a while but then didn't score.
"We have to realise where we are, what we have and where we want to be. I've said before it is going to be a tough season and we have to keep working as hard as possible."
Next up for Newcastle is a trip to West Bromwich Albion on Tuesday evening.