The Hornets suffered a miserable afternoon at Vicarage Road on Saturday, but Boothroyd has tried to rally his troops by talking up their hopes of mounting a comeback.
"I said to the players that it is half-time and that is all it is," he said.
"We've come back from 2-0 before many, many times and we've got to make sure that we go and give it a right good go because they think they've won.
"We're the best away team in the league and I was delighted with so many things from the players.
"I thought we played with freedom and we expressed ourselves. I thought we showed a lot of courage and bravery.
"We are disappointed, of course, but we've got to be smiling inside because we know if we get one goal up there in the first ten minutes then the tie completely changes."