Frank Lampard became the first Chelsea manager to lose on his Champions League debut as his side fell 1-0 to Valencia on Tuesday.
The Group H game at Stamford Bridge was scoreless until Valencia forward Rodrigo got on the end of a free-kick in the 74th minute to give the away side a 1-0 lead.
Chelsea would have a golden opportunity to level the match in the 87th minute, but substitute Ross Barkley blazed over from the spot to ensure his side would go down to a home loss.
Barkley was the first Englishman to take a penalty for Chelsea since Lampard himself in April 2014 against Stoke City, with both players missing from the spot.
The defeat saw Lampard become the first of 11 Chelsea managers to lose on their Champions League debut. Ten of those managers won in their first game, with Gianluca Vialli drawing in his first match in 1999.
Tuesday also marked only the second time that Chelsea have lost their opening Champions League match of a campaign, with the Blues also losing in 2013-14 against Basel.
It was an inauspicious start to life in European competition for Lampard, who took over Chelsea in the off-season after Maurizio Sarri left the club to join Juventus.
Lampard has had mixed fortunes in his first two months as Blues manager, having started Chelsea's Premier League campaign with a discouraging 4-0 defeat at Manchester United.
The Blues have rallied in the Premier League somewhat after that loss, winning two and drawing two of their next four games.
With the club under a two-window transfer ban, the play of Chelsea's academy products has been a particular highlight of the season.
Mason Mount, Tammy Abraham and Fikayo Tomori have all performed well under Lampard – particularly Abraham, who is the co-leader for the Premier League's Golden Boot with seven goals thus far.
Mount has also shown well but his immediate future is in doubt after he was removed from Tuesday's game with an ankle injury.
Chelsea's challenge will not get any easier after Tuesday's game, with European champions Liverpool next up in a Premier League fixture on Sunday.