Chelsea couldn't believe that Arsenal were so eager to sign David Luiz in the last transfer window, according to Tony Cascarino.
Luiz's second spell at Stamford Bridge came to end on August 8 when he completed an £8 million ($11m) switch to Emirates Stadium.
The Brazilian has always been error-prone, but his final year at Chelsea was arguably his worst yet, and Frank Lampard made it clear that he would not be in his plans for the 2019-20 campaign.
Arsenal might have hoped that a Champions League winner with bags of experience would help solve their defensive issues, but he has so far failed to endear himself to the Emirates Stadium faithful.
The Gunners have slipped to 11th in the Premier League table after 19 fixtures, eight points behind fourth-place Chelsea with 28 to their name in the goals-against column already.
Cascarino says that the Blues were surprised Arsenal wanted to bring in "their poorest defender", and jumped at the chance to sell Luiz to a direct rival for a cut-price fee.
When asked to name the worst signing of the summer in his latest column for The Times, the ex-Chelsea striker wrote: "David Luiz.
"When Arsenal bought the Brazil defender in the summer for £8m I know that Chelsea’s reaction was: ‘Wow, they’ve taken our poorest defender’.
"You have that familiar image of him chasing after a forward who has got behind him: opponents always know that the 32-year-old will give them space by wandering out of position.
"He is a fine footballer who sprays the ball around well but for a player with so much experience his defending is not good enough."
Luiz will likely retain his spot in Arsenal's starting line-up when they come up against his former employers at the Emirates on Sunday.