The Arsenal legend referred specifically to the Brazilian’s performances in the FA Cup final, where Chelsea were beaten 2-1 by the Gunners.
Alexis Sanchez broke the deadlock at Wembley before Diego Costa equalised for the Stamford Bridge club after the break.
Chelsea looked like they could be getting back in the clash, despite being a man down, but Aaron Ramsey responded just moments later.
The Welshman launched himself at a Olivier Giroud cross, heading past Thibaut Courtois from close range to win Arsenal the game.
And Merson believes this was an indication that Luiz, who was in the box when the ball was crossed, isn’t as good as people are suggesting.
“I think Conte needs to bring in another defender,” Merson told Sky Sports.
“I know everybody goes on about how good David Luiz has been but he got found out again in the FA Cup final.
“He had no awareness of the danger for the Aaron Ramsey goal.
“They looked tired in the cup final so they definitely need some more numbers in there to be able to balance Champions League and defending the Premier League title, especially with everybody out to beat the champions.”