Ross McCormack scored an injury-time winner to ensure Watford missed out on automatic promotion, with Hull going up.
The Hornets will have to try to join them via the play-offs.
"It was an amazing game," McDermott said.
"Obviously we knew what was going on at Hull but we did what we had to do. I had a feeling Ross McCormack would make a difference today.
"Gianfranco is a friend of mine and I feel for him but during the game you have to do what you have to do.
"He's a top person who has done a fantastic job, and they still have every chance of going up."