Isaac managed to get a goal in the 2-0 win over Reading, which ensured his side progressed into the next round.
And the striker is so happy that he can finally make an impact, after two years without a goal.
"It felt so great," he said to theWatford Observer.
"My teammates all celebrated with me and they were so happy to get that first goal.
"That made me comfortable and relaxed and helped me through the game. I am happy to get my first 90 minutes and hopefully I can get more.
"I think physically I am better now, so I wasn't tired after 90 minutes."