The Wizards were 22 points down at half time in their 99-96 defeat and will face the Pistons again in their opening game of the regular season.
The head coach told reporters: "We did not show up. Our guys think they can turn it on and off like a light switch and we are kidding ourselves.
"We came out and did not play with each other and why we continue to fall back? I have to figure that out. That was an embarrassment.
"In the second half they played for each other, got after it, and made it a game. It is pretty black and white, but we will not for whatever reason buy into that."
Washington play the Cleveland Cavaliers in their final pre-season game later today.