Quique Flores feels Newcastle United are a "dangerous" opponent – despite their lowly league position.
The two teams meet at Vicarage Road this afternoon for the second time in a fortnight.
Troy Deeney scored the only goal of the game two weeks ago when they met in the third round of the FA Cup.
Newcastle were the better team that afternoon, but their finishing let them down and they were beaten 1-0 for the fourth successive game.
And Watford head coach Flores has acknowledged that his side were "lucky" to win the tie.
"Newcastle is a fantastic team," said Flores in his programme notes.
"They showed how strong they were during the cup game, and we know we were a little bit lucky to get through to the next round.
"Our defensive line was good, but we were under a lot of pressure during the match.
"They knew about taking chances and we won the game because we did that and they couldn't do this, but their results and performances since then show how dangerous a team they are."
United signed Jonjo Shelvey and Henri Saivet last week.
And Flories added: "There are new players for the coach (Steve McClaren) to choose as well, just like here at Watford, so it'll be a different game in many ways."