The forward thumped the ball out of the hand of the kid so as to quickly restart play as the Hornets desperately searched for a draw in the tie.
The Nigeria international was subsequently cautioned by referee Graham Scott for the incident and the Spanish tactician has frowned at the reaction from the striker.
"I have my opinion about Isaac's behaviour. I don't like to see that from my player with a ball boy," Gracia told Sky Sports.
"I think we have to look after the ball boys always. It is not a good reaction. That is what I can say.
"I am not here to educate the ball boys on what they have to do and not to do - it is not my job. We need to improve our behaviours next time."
Success who made his 27th appearance in the encounter has scored four goals in all competitions this season.
Watford are placed ninth in the league standings with 33 points from 24 games. They take on Brighton & Hove Albion in their next league game on Saturday.