Chelsea advanced to the FA Cup quarter-finals after a 3-2 win at Kenilworth Road but found themselves behind twice in the first half.
Reece Burke and Harry Cornick scored either side of Saul's equaliser and Sarr, who was given a rare start by manager Thomas Tuchel, played a part in both goals.
It saw Shearer criticise the 23-year-old for being 'too nice' while defending the opener.
"Sarr’s marking Reece Burke, meant to be… he’s too nice isn’t he, there’s not enough aggression there," Shearer explained on BBC.
"He’s a yard off him, he touches him, not enough.
"It’s a very good ball in and run and the contact is superb the way he guides it in that far corner."
Richards was also in the studio and agreed with Shearer's analysis.
The former Manchester City defender added: "Like you say, once he’s got the run on him and he’s got the height on him as well… he’s not aggressive enough.
"Sometimes if you’re a smaller guy you’ve just got to make sure you’re close to someone, stop him from being able to get that run.
"It didn’t happen and he’s cost his team a goal."
Burke scored inside two minutes before Saul got the Blues back in it just before the half hour mark.
Sarr then played Cornick onside after stepping up too soon and it allowed the Hatters forward to run on and put past Kepa Arrizabalaga.
The France U21 international moved to Stamford Bridge from Nice in 2020 but spent a season on loan with Porto.
He's featured 16 times in all competitions this campaign.
Timo Werner and Romelu Lukaku scored in the second half to ensure Thomas Tuchel's side didn't slip up against Championship opposition, on the day Roman Abramovich announced his intentions to sell the club.
Chelsea have reached the FA Cup final in the last two seasons but lost to Arsenal and Leicester City in 2020 and 2021 respectively.
They're on a run of five cup final defeats at Wembley in a row after losing the Carabao Cup final to Liverpool on Sunday.
The Blues sit third in the Premier League and return to action against Burnley on Saturday afternoon.
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