The League One high-fliers were dumped out at the area quarter-final stage to lower division opposition after losing 2-0 at the Don Valley Stadium.
"I am disappointed. That was probably our worst performance of the season," he said.
"We didn't perform well enough to win the game - it is as simple as that.
"We gave two very poor goals away. When you do that it is fair to say you are going to give yourselves quite a lot to do.
"I don't think we showed the type of drive that we needed and that is very disappointing.
"This has been a missed opportunity - there is no doubt about that. You don't get second chances in football."