The midfielder, who has been an almost ever present this season for the Bucks notched a late penalty.
But at that point his side had shipped three against relegation threatened FC United of Manchester.
Deeney said the side were not at it today, but complacency had nothing to do with the defeat.
He said: "We were just not at it.
"Defensively, from back to front it was just one of those days we have to put to bed and go again.
"I can't think of any reason for the performance, we tried to do the right things and it was just not coming off.
"It was a bad day at the office."
When asked if complacency had crept in he said: "No, we know we need points to cement a place in the play offs.
"We know every game is a big game. We don't do that, it is not our mentality.
"It was just not a good day at the office."
The Bucks performance will have to go up if they have any chance of getting something from the first leg of the FA Trophy against Leyton Orient next week.
Deeney admitted the intensity in training will now increase, with everyone vying for a place in the starting side next week.
He added: "I think it is one of them games where have hunger anyway.
"It is massive and everyone wants to play.
"It will be tense throughout the week with everyone trying their best to get in that side."