"I was pleased with our attacking play, but not some of the defending. It was schoolboy stuff," admitted Boothroyd.
The home side were 3-0 down at half-time, but dragged the scoreline back to 4-2 thanks to goals from Damien Francis and Danny Shittu.
The Hornet's boss admitted that his side definitely had the correct attitude, but the mistakes cost them the points.
"I don't think it's psychological. I might have done before today, but the attitude and application is all there. But individual defensive errors you just can't legislate for."