The Blues were tipped to challenge for the Premier League title this season, but have endured a poor campaign and are in danger of failing to qualify for the Champions League next year.
Lampard recognises the current squad have not performed to the high standards expected of them, despite notable highlights including the superb 5-4 aggregate win over Napoli earlier this month.
A midweek defeat to Manchester City and a 0-0 draw with Tottenham Hotspur at the weekend leaves them five points adrift of their London neighbours in the race for fourth place.
"The fixtures and where we are in the table doesn't lie - we're not as good as we used to be," the England midfielder told Chelsea TV.
"We used to have a fortress here and, for whatever reason, we're just not doing it. It's something we must all put right.
"You know a derby against a very good Tottenham side is going to be tough. But I don't think we were at it enough. We had to play more with the urgency we showed against Napoli.
"Against Tottenham, we didn't really do enough to lift the fans.
"We had the high of Napoli, then we got beaten by Manchester City and now we need to up it again."
Chelsea have qualified for the Champions League for nine successive seasons, and Lampard has challenged his Blues team-mates to keep the run going.
He added: "Chelsea should be in the top four. No disrespect to the other top-five clubs but, with the ability we have, we should be in there.
"It is far from over. There is no point being downbeat about it at the minute.
"It's five points and the gap has been bigger than that. It's up to us now to get ourselves right for Benfica and the league games coming up."
