The Blues were knocked out of the cup competition on Tuesday night as Jose Mourinho’s Spurs booked their place in the quarter-finals with a penalty shootout win.
Summer signing Timo Werner gave Chelsea the lead in north London but Erik Lamela’s late equaliser took the game to penalties and Mount missed the target after nine successive converted spot-kicks.
Lebouef believes Mount was ‘awful’ throughout the match and says he and his team-mates need to ‘wake up’ if they want to enjoy careers at Stamford Bridge.
‘We saw so many defensive mistakes from Chelsea last season. And they still have work to do,’ the former Chelsea and France defender told ESPN FC.
‘But it’s not just the defenders, it’s the midfielders too, they have to work and come back.
‘Mason Mount was awful all game. He’s a very talented player but some players have to wake up if they want to have a career.
‘A career is not only six months or a year, it has to be more.’
Leboeuf, who played more than 150 matches for Chelsea, also said it was ‘sad to see’ his former club struggling for consistency despite their spending spree.
The Blues have spent more than £200m this summer on a host of new signings but have nonetheless made a slow start to the new season.
‘Chelsea were very good in the first half and were dominating the game. They put Tottenham under pressure and scored the goal,’ he added.
‘Credit to Frank and Chelsea for the first half. But I’m very critical for their performance in the second half.
‘It’s sad to see that that team is still incapable of playing well for 90 minutes.
‘They have to change that, otherwise it’s going to be a problem for them.’
Chelsea face London rivals Crystal Palace on Saturday ahead of the first international break of the 2020-21 campaign.