Benitez told the club's official website: "United are top of the table and we were better than them in the first game. We can compete, we have played some nice football, scored a lot of goals, but still, we have to be more consistent.
"We have offensive players with enough quality to create, but in some positions to find the balance is not easy.
"To play two games a week for four months is not easy to keep the team at the same level the whole time."
Chelsea have an excellent record in the FA Cup in recent years, winning the trophy four times in the past six seasons.