Phil Neville has blasted Chelsea for their decision to sell Nemanja Matic to Manchester United in the summer after Tuesday night's 3-0 defeat against Roma.
Antonio Conte's side were comprehensively beaten in the Italian capital and now sit second in their Champions League group.
Chelsea may have been given a boost by Atletico Madrid's unexpected failure to beat Qarabag, but their performance has been heavily criticised and Neville did not shy away from expressing his opinion.
"What's gone wrong for Chelsea? One word. Matic," he told BBC 5Live. "It's as simple as that. The minute they sold Nemanja Matic was a mistake.
"When you had Matic sat in front of you, alongside [N'Golo] Kante, there was protection. Whoever made that decision needs sacking. That is one of the poorest decisions I have ever seen in the Premier League.
"If you ask any of those Chelsea players who they would want back, they would say Matic. Chelsea replaced him with someone who is going to do the same job – [Tiemoue] Bakayoko is not that player, he is not a holding midfield player.
"They didn't replace Matic. I wouldn't have let him out of the building, I would have chained him to the training ground walls."
Matic will return to face his former teammates on Sunday as United travel to Stamford Bridge in the Premier League.
The 29-year-old has started all ten of his side's league games so far this season, impressing in Jose Mourinho's midfield.