"We were devoid of any ideas and legs and it was one of the worst defensive performances I've seen," said the Scot. "It was a brain-dead performance.
"There needs to be a vast improvement because we won't get to the playoffs with displays like that. I didn't think Watford were extra special - just that we were very poor on the day.
"We know we can do a lot better than that so I've got to pick the players up on Monday."
Forest currently sit 10th in the Championship table, six points below the final playoff place.