Eustace: Will feature at KC Stadium
Eustace was sent off after an altercation with Tigers skipper Ian Ashbee during Sunday's 2-0 defeat at Vicarage Road but has had his red card rescinded after an appeal to the Football Association.
Boothroyd will also be boosted by the return of top scorer Darius Henderson from suspension as he seeks only a second win in 16 games to complete a remarkable comeback.
Henderson's return will give Boothroyd a tactical dilemma after winger Tommy Smith excelled in a free role behind lone striker Nathan Ellington in the first leg. Steve Kabba remains out with his ankle injury.
Defender Danny Shittu is a serious doubt after he strained his hamstring against Hull so former captain Jay DeMerit is poised to return alongside Leigh Bromby.