The home side made a bright start to the match, but neither side could find their rhythm early in the first half, as injuries interrupted the game.
Watford goalkeeper Jonathan Bond was stretchered off after 24 minutes after sustaining a suspected neck injury, resulting in 16 minutes of stoppage time at the end of the first period.
The visitors took the lead in the 41st minute, as Dominic Poleon tapped the ball in from close range.
Nathaniel Chalobah had a chance to equalise moments later, but it was Almen Abdi who got the Hornets back on level terms five minutes into injury time with a low, powerful strike from just outside the penalty area.
The home side made a strong start to the second half, but found themselves down to 10 men when top goalscorer Troy Deeney picked up his second yellow card in the 60th minute.
The result means that Watford will have to go through the playoffs if they are to secure a place in next season's Premier League.