Goalkeeper Egil Selvik says Watford's young team must "grow up fast" after starting the new season with defeat at promoted Charlton Athletic.
The Hornets were set pick up a point in Paulo Pezzolano's first league game in charge when Harvey Knibbs' added-time goal gave the home side victory.
Watford only managed one shot on target during the game despite having the lion's share of possession at The Valley.
"Losing the game at the very end is always painful - but it's also an opportunity to learn," Selvik told BBC Three Counties Radio.
"On the goal, we need to grow up fast.