The Hornets head into Saturday's FA Cup final clash with Premier League champions Manchester City on a poor run of results - but Deulofeu insists anything is possible in a one-off game at Wembley.
"We started really badly, conceding the two goals," the Spaniard said.
"We have to be stronger. We have to be more aggressive, more angry.
"It is one game against City and anything can happen. We know our characteristics - we will fight with all our heart and as hard as we can for the trophy.
"We will fight all the way through what will be an amazing battle. It is a final and we have the opportunity to reach Europe and make history. We will go full power in this final game of the season."