Jamie Redknapp says West Brom exposed a fatal flaw in how Chelsea set-up to defend corners in their game at The Hawthornes.
The Baggies raced into a 3-0 first-half lead in a dismal opening 45 minutes for Chelsea, who made numerous defensive mistakes.
Kyle Bartley added the third goal from a corner and Sky Sports pundit Redknapp said it came from a weakness in how Chelsea defend set-pieces.
He said: "The third goal comes and I think you have to give West Brom credit because they've looked at Chelsea's set-up on corners.
"They've realised that they put everyone back but they don't have a man on the edge of the box.
"Jake Livermore gives a signal that everyone is going to go into the box from West Brom's point of view and then it's going to leave the edge of the box free.
"It's good timing, a good header and he puts it into an area hoping someone can get on the end of that second ball.
"You can see it just works perfectly. Livermore comes in just in case there is a problem and they lose it but it's a good header at the far post and Bartley just slips it into the goal.
"It shows you how West Brom have been so switched on and Chelse haven't."
He added: "It's a dangerous game not having anyone on the edge of the box, not only for that reason but if the ball comes back out, someone gets a free shot at goal.
"I don't like that set-up."
Former Baggies midfielder Gareth Barry agreed with Lampard, revealing it is something Slaven Bilic's side would have worked on.
Barry said: "They'll have looked at the previous corners Chelsea have defended.
"It's quite strange they've got all the players back but they've still got room on the edge of the box to find a man.
"They'll have definitely picked up on that and, once you get that first contact, the players in the area are on their toes.
"The Chelsea players are ball-watching whereas the West Brom players know it's coming and Kyle Bartley tucks it in."
