Benitez said: "We had more determination in the second half, but the first half was the same kind of performance away from home as last year.
"Everyone must understand these kinds of games are important. It's acceptable to play at the same level as Chelsea and lose, but it's not acceptable to play Fulham, whom we know we are better than, and come away with nothing.
"It was as though we didn't understand the importance of the game in the first half. Only in the second was there a response, and then I felt it was a question of time whether we would score, but their keeper made a lot of saves.
"Last season we made a lot of mistakes and I thought we were learning from them. We've started strongly in other away games and I didn't expect to see us play like this again after last year.
"This is our first defeat away this year and we can't afford to make the same mistakes in future. It's important the players understand we can't approach games in this way.
"It's not just the players who need to improve, but all of us. I include the staff, the players, everyone.
"We all know we need to change the attitude, particularly after European games. Yesterday we all spoke and watched the match again. The performance of the team was bad in terms of the aggressiveness of the players.
"We had a lot of the ball, and controlled the play, but didn't show enough until the second half. We've had these problems before after a Champions League game.
"We've all seen the big mistake we made with our first half performance and now we need to correct it."