Stephens determined to get back to basics raced into a 2-0 lead after a brace from Glenn Murray but they left Craven Cottage with nothing after a disappointing second-half display.
They conceded four goals as Fulham boosted their chances of survival and moved nine points behind Chris Hughton’s men.
Stephens concedes his team let themselves down but he has urged them to move on ahead of the meeting with Watford.
He told the club’s official website: “It thought we played well in the first half and deserved our lead at half-time. But we then went away from what we were doing in the first half. “They’ve got a lot of quality in their team and they showed it after the break. It was all our own undoing in the end. “It was the basic stuff, having a higher line, keeping the ball — everything we did in the first half to get the lead. When you go away from doing those things, especially away from home, you’re going to get punished.”