His opposite number Gianfranco Zola was left to rue a goalkeeper crisis as Watford prepare for a two-legged play-off semi-final with Leicester.
Zola was forced to hand a professional debut to 19-year-old Jack Bonham against Leeds after injuries to first-choice goalkeeper Manuel Almunia and back-up Jonathan Bond.
Almunia felt his hamstring in the warm-up and Bond, his replacement, was stretchered off following a sickening collision in the 22nd minute.
Unsurprisingly, Bonham looked nervous when he came on and he was soon picking the ball out of the net after a mix-up with Joel Ekstrand allowed Dominic Poleon to score for the visitors.
The Hornets levelled through Almen Abdi’s superb curled shot during 16 minutes of stoppage time at the end of the first half, and as news of Cardiff’s late equaliser against second-placed Hull filtered through, Zola’s side knew another goal would send them up.
However, as they piled forward Leeds broke at the other end and Bonham let Ross McCormack’s lob squirm through his hands and roll into the back of the net to condemn Watford to a 2-1 defeat and a place in the play-offs.
“It was an amazing game,” said McDermott.
“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.
“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 done a fantastic job, and they still have every chance of going up.”
Zola said he was not going to blame teenager Bonham “for one second”, adding: “It was tough to come in at that moment – one minute he was in the stands, then on the bench, and then on the pitch.
“In football you make mistakes and it is how you react that is important.”
After 10 minutes of treatment on the pitch Bond was taken to hospital in a neck brace and oxygen mask, although mercifully it seems he suffered nothing more serious than a broken nose.
As well as fitness concerns over his goalkeepers, Zola will also be without Troy Deeney for Thursday’s play-off first leg after the striker was shown a second yellow card.
Watford: Bond (Bonham 24), Doyley, Cassetti, Ekstrand, Hogg (Yeates 83), Pudil, Anya (Vydra 64), Chalobah, Abdi, Deeney, Forestieri. Unused substitutes: Hall, Briggs, Geijo, Battocchio.
Leeds United: Kenny, Peltier, Lees, Pearce, Drury, Green (Hall 90), Tonge, Brown (White 64), Varney, Morison (Poleon 10), McCormack. Unused substitutes: Cairns, Warnock, Killock, Lenighan.
Referee: Graham Salisbury (Lancashire).