Paul Merson believes Frank Lampard made a mistake allowing David Luiz to leave Chelsea.

Luiz joined Arsenal on deadline day and Merson thinks Chelsea will be "suffocated" by teams without the Brazilian's ability to play out from the back.

Speaking after the Blues were beaten 4-0 by Manchester United on Sunday, Merson said: "I go to Stamford Bridge a lot, and Chelsea's defence are asked to play out nearly all the time," Merson said to Sky Sports.

"Kepa doesn't kick anything long. Luiz was the best player on the ball by a million miles. Now all of a sudden, you've got players who can't play out, and teams will suffocate them.

"Frank Lampard has got to be careful. Chelsea have let their best ball-playing defender leave to go to their rivals, not just in London but in the top four. None of their defenders can play out from the back.

"I can understand if Chelsea pushed everyone up to the halfway line, the goalkeeper kicks it, and they play in the other half, but if you're asking them to start playing out from the back, they're going to get beaten left, right and centre.

"Kurt Zouma just didn't look ready. He didn't look ready before this, which is why he went out on loan. It's a dangerous, dangerous game. The fans liked David Luiz. They need to get results quickly, because the fans won't be happy watching them play out and lose the ball.

"It looked like Frank looked at the situation and thought: 'I need it quiet here, I can't be having problems', so Chelsea let Luiz go. Sometimes that's the way it is, and great managers have a high-maintenance player and get them to perform."