City were on course for a home tie with Spurs thanks to Wade Elliott's second-half goal, but Leeds responded through Ross McCormack and El-Hadji Diouf to beat their hosts and reach the fourth round.
The result means Birmingham miss out on a glamour game and its much-needed attendant revenue, and Lee Clark's assistant was sent out to face the press afterwards.
McDermott said: "Of course we're disappointed, we're very disappointed with the second-half performance, no one more so than the manager.
"It is a big disappointment because I'm sure the fans would have loved to have seen Tottenham playing here. We're disappointed. I think we're a bit more than disappointed actually, we're devastated really because we wanted a big game like that.
"Obviously it would have helped the club as well financially, but it's not to be.
"But, for me, the most important game is on Saturday against Brighton and getting three points and putting a performance on."