The AFC East were comfortably beaten at FedEx Field by the Washington Redskins to lose their 100% pre-season record.
Speaking at a press conference, the head coach said: "Obviously, we are disappointed in our performance. We had a chance to make some plays early on in the game and didn't. They, in turn, did and were able to score.
"I think at the end of the day, we have to find a way to make those plays and take advantage of that opportunity. We have to continue to keep working on the fundamentals. As I said before, camp is not over.
"It is a work in progress and we have a lot of work ahead of us. I think we left some plays out there. We have to just go out there and make some plays."
The Bills face the Detroit Lions in their final game of the pre-season on Thursday.