Rafa Benitez has questioned whether Salomon Rondon would have made a difference against Watford – as his team wasn't creating enough chances.
Rondon was an unused substitute in Saturday's 2-0 FA Cup defeat.
Benitez, keen to "protect" players ahead of tomorrow night's home game against Manchester City, opted to bring defender Fabian Schar on with nine minutes left as his team, trailing 1-0 at the time, chased the game.
The decision to sent on Schar ahead of striker Rondon was questioned by some fans.
Asked about leaving Rondon, the club's leading scorer, on the bench, Benitez said: “You have to create chances to use the striker, you cannot just blame the people up front.”
United's manager added: "We didn't create too many chances – it (the defeat) was not becayse of the strikers."
Benitez had fielded Joselu – who scored in the club's replay win over Blackburn Rovers – up front.
Newcastle had just one shot on target at St James's Park.