The Bees were stung by the Hornets in a behind-closed-doors friendly on Wednesday, with Robson using the game as a chance to run the rule over several trialists.
Robson was unhappy with the way his side pressed their Championship opponents but was pleased with their second half showing.
"We didn't look as sharp as we have been throughout pre-season and we can learn some valuable lessons from this game," he told the club's official website.
"We need to look at how we press the opposition as a team, rather than individually, something that Watford did very.
"The second half looked a lot better and we defended well as a team. Some changes were made by both teams and we made sure that we made it harder for them. It certainly gives us lots to think about for the immediate future."