Havertz was a second-half substitute as he returned to action after suffering a knee injury on the opening day of the season against Manchester United.
Havertz said: “It’s a special feeling, it’s amazing for myself. I'm just happy to be back with the boys and I hope I can help them as much as possible now.
“I think the way the boys have played the last couple of months, it was so nice even to watch from the outside, even though it's always very hard to be in a situation like this. But just to watch them was amazing and I'm very happy to be with them now.”
The Germany international also commented on Gabriel Martinelli's hat-trick in the 4-1 win.
He said, “He’s buzzing. I think he deserves it. Every day he works so hard, and especially the way today, with two goals after a set piece, it's incredible. So I think we're all very happy for him.
“They were top, especially against teams where they actually focus a lot on set pieces, so to jump that high and then score two goals is amazing.
“I think it's always tough in the third round of the FA Cup, and a place like this is always tough to play at. We didn't play our best football, but we won the game and that’s the most important thing.”