Patrick Vieira is still waiting for his first win and goal as Crystal Palace manager as a late goal by debutant Ashley Fletcher guided Watford into the third round of the Carabao Cup.
Vieira's side dominated the game and striker Jean-Philippe Mateta saw two first-half efforts saved by Ben Foster.
Foster also denied Christian Benteke while midfielder Cheikhou Kouyate hooked over the bar.
Fletcher sealed the win by stretching to turn home a Cucho Hernandez cross.
With a week of the summer transfer remaining Vieira, who is managing in the Premier League for the first time, might be looking for a new centre-forward.