Willian’s brace was not enough as goals from Tomas Soucek, Michail Antonio and Andriy Yarmolenko condemned Chelsea to defeat at the London Stadium.
Lampard’s side now sit just two points above Manchester United and Wolves with six games remaining.
Lampard said after the match: “I think when a team is bigger than you, you can work on it as much as you want, but if you are much taller and they outjump you then you will concede goals.
“We were aware of it today but the players have to deal with it on the pitch.”
He added: “It’s a sign of where we are – we have a lot of hard work to do to get to where we want to be.
“The reason the rest of us are chasing Liverpool and Man City is because of the consistency they have developed so that is what we have to work towards.
“It’s obviously frustrating but if we are in that position it’s how you bounce back and move forward.”
He then added: “I’ve not been let down by my defenders – we’ve made mistakes in the buildup to the goals, not just defenders, which cost us the game.
“I wouldn’t call it a wake-up call because every game is different. This wasn’t a huge surprise – if you know the Premier League and you make mistakes, it can happen. We have to accept it and move on.”