Eustace was sent off by referee Kevin Friend for a clash with Hull skipper Ian Ashbee at Vicarage Road in the 2-0 first-leg defeat but has had his red card rescinded after an appeal to the Football Association.
After the game, manager Adrian Boothroyd said: "With the sending-off we have a situation where the Hull players are more experienced than the referee and they've talked him into a red card.
"The ball's on the floor, there are bodies around and he's gone for the ball.
"Are we not allowed to tackle any more?"
Watford last week