London - Chelsea manager Frank Lampard wants his young side to be more ruthless after a 2-1 win over a much-changed Lille at Stamford Bridge secured their place in the last 16 of the Champions League.
Progress to the knockout stages at the expense of last season's semi-finalists Ajax is an impressive achievement for the Blues boss after a summer in which the club lost Eden Hazard to Real Madrid and were unable to add to their squad due to a transfer ban.
First half goals from Tammy Abraham and Cesar Azpilicueta seemed to have Chelsea cruising into the knockout stages.
However, Lampard was left frustrated at having to endure a nervy final 12 minutes when Loic Remy pulled a goal back against his old club.
"It's going to be a work in progress. With the young players we have in this side there will be bumps in the road," said Lampard.
"I push them and can be quite hard on them, setting the high standards I want to see them reach, but at the same time I have to be patient.
"Today was a sign of how much work we still have got to do. It should have been a calmer finish to the game and that is the biggest thing that we need to improve."
Chelsea progress in second place in Group H behind Valencia, who beat Ajax 1-0 in Amsterdam, meaning they face the daunting prospect of a last 16 tie against Bayern Munich, Barcelona, Paris Saint-Germain, Juventus or Leipzig.
"It's a really good achievement for the group, but in the next stage and how cut throat it will be, we can't afford to miss chances or give chances away like we have," added Lampard.
"It's a very tough draw no matter who it is because we came second in our group.
"We should be very pleased we are there and embrace the challenge whoever we get."
Chelsea had won just one of their previous five games after a great start to life under Lampard and he responded by adding a little more experience to what has been a very young side so far this season.
Antonio Rudiger returned from injury to make just his second start of the campaign at centre-back, while Jorginho was restored in midfield to the side that lost 3-1 to Everton on Saturday.
Up front, Abraham was again flanked by Christian Pulisic and Willian, and all three combined to open the scoring on 19 minutes.
Pulisic unselfishly played in Willian on the right instead of opting to go for goal himself and the Brazilian's driven cross was tapped in by Abraham for his 13th goal of the season.
Lille have been the whipping boys in a three-way shootout for two places in the last 16 and showed why 10 minutes before half-time when Azpilicueta was left completely unmarked to head home Emerson's corner from close range.
Mike Maignan in the Lille goal kept his side in the game with a fine double save from Pulisic and Willian at the start of the second half.
However, Chelsea took their foot off the gas as Lampard replaced Pulisic and Abraham ahead of a busy festive period in the Premier League.
And the hosts had to suffer a nervy final dozen minutes when Remy fired in off the underside of the bar.
Remy was a peripheral figure during three seasons at Chelsea between 2014 and 2017, but nearly came back to haunt his old employers only to fire weakly into the arms of Kepa Arrizabalaga in stoppage time.
"We need to take more of our chances because you leave teams slightly in the game," added Lampard.
"They had two shots and one goes in. That was the frustrating thing in a very good performance."